Medical examiners say the initial portion of their autopsy on Bobbi Kristina Brown has found no obvious cause of death or significant injuries.

The Fulton County Medical Examiner's Office began its autopsy Monday on the daughter of singer Whitney Houston and said there was no evidence of previously unknown medical conditions. Brown died Sunday in hospice care nearly six months after being found unresponsive in a bathtub inside her suburban Atlanta townhome.

Authorities say passage of time will make it more difficult to determine how and why Brown died.

The medical examiner's office says it is ordering lab tests and has issued subpoenas for documents - likely Brown's hospital records - to help determine a cause and manner of death. The agency says a ruling should take several weeks.

Bobby Brown 'completely numb' after death of daughter

R&B singer Bobby Brown said Monday that his daughter, Bobbi Kristina Brown, was "an angel" and that the family must find a way to honor her memory.

Bobbi Kristina, the daughter of singer Whitney Houston, died Sunday in hospice care, six months after she was found face-down in a bathtub in her suburban Atlanta townhome. Authorities are investigating her death.

"Krissy was and is an angel," her father said in a statement. "I am completely numb at this time. My family must find a way to live with her in spirit and honor her memory. Our loss is unimaginable."

Bobbi Kristina had been hospitalized for months in Atlanta - eventually being placed in hospice care - after being found in a manner grimly similar to the way her megastar mother died three years earlier.

A police report earlier this year described the incident as a drowning. In a statement Monday, the Fulton County Medical Examiner's Office said an autopsy will be needed to evaluate what led to the death of Bobbi Kristina, but the time that has elapsed since the autopsy will complicate the investigation. The medical examiner did not release a timeframe for the autopsy, but said additional lab testing might take several weeks.

"Interpretation of autopsy findings and other information will also be challenging," the medical examiner's office said. "However, an autopsy could be helpful to address questions which may arise about the cause of her unresponsiveness and eventual death."

Bobbi Kristina was the only child of Whitney Houston and R&B singer Bobby Brown.

"She is finally at peace in the arms of God. We want to again thank everyone for their tremendous amount of love and support during these last few months," Kristen Foster, a representative for the Houston family said Sunday.

Nick Gordon, who shared her townhome with her, said at the time it seemed Bobbi Kristina wasn't breathing and lacked a pulse before help arrived.

"The Roswell Police Department continues its investigation into the circumstances preceding and surrounding the time of the original incident leading to her death," the medical examiner said.

Born and raised in the shadow of fame and litigation, shattered by the loss of her mother, Bobbi Kristina was overwhelmed by the achievements and demons of others before she could begin to figure out who she was.

Bobbi Kristina - the sole heir of her mother's estate - did have dreams.

She identified herself on Twitter as "Daughter of Queen WH," "Entertainer/Actress" with William Morris & Co., and "LAST of a dying breed." She told Oprah Winfrey shortly after her mother's death in 2012 that she wanted to carry on her mother's legacy by singing, acting and dancing. But her career never took off.

She became a social media sensation, sending more than 11,000 tweets and attracting 164,000 followers.

As the news of her death spread across social media, several celebrities tweeted their condolences.

Grammy-winning performer Missy Elliot tweeted, "My heart is truly heavy. May u rest in peace with your mommy (hash)BobbiKristina sending prayers 2 the Brown/Houston family."

"Empire" star Taraji P. Henson tweeted, "Rest in heaven."

And Winfrey tweeted, "Peace at Last!"

Whitney Houston, known as "America's Sweetheart," was an impossible act to follow.

The late singer sold more than 50 million records in the United States alone during her career. Her voice, an ideal blend of power, grace and beauty, made classics out of songs like "Saving All My Love For You," "I Will Always Love You" and "The Greatest Love of All." She earned six Grammys and starred in the films "The Bodyguard" and "The Preacher's Wife."

Bobby Brown, who had a bad-boy image, also became a huge star, selling platinum records with New Edition and going solo before drugs and legal woes derailed his career.

Bobbi Kristina appeared alongside both parents in 2005 on the Bravo reality show "Being Bobby Brown," which captured her parents fighting, swearing and appearing in court. The Hollywood Reporter said it revealed that Brown was "even more vulgar than the tabloids suggest," and managed "to rob Houston of any last shreds of dignity."

After their divorce in 2007, Houston kept custody of Bobbi Kristina and raised her alongside Gordon, an orphan three years older than her daughter. Houston brought Gordon into her family, and while she never formally adopted him or included him in the will, both teenagers called her "mom."

The threesome's tight bond was shattered when Houston's assistant found the singer's lifeless body face-down in a foot of water in her bathtub at the Beverly Hilton Hotel just before the Grammy Awards in 2012. Authorities found prescription drugs in the suite, and evidence of heart disease and cocaine in her body, but determined her death was an accidental drowning.

Bobbi Kristina, then 18, was at the hotel and became so hysterical she had to be hospitalized. "She wasn't only a mother, she was a best friend," she told Winfrey.

She and Gordon then went public with their romance, posting defiant messages online after the tabloids accused them of incest.

Relations between Gordon and some other relatives soured over the past year after Bobbi Kristina was hospitalized. A protective order barred him from being within 200 feet of Pat Houston, Bobbi Kristina's aunt. A feud erupted over whether Gordon could visit Bobbi Kristina while she stayed in the hospital.

On June 24, Bobbi Kristina's court-appointed representative sued Gordon, accusing him of misrepresenting his relationship with Bobbi Kristina. The complaint accused him of being violent toward her and taking more than $11,000 from her account while she was in a medically induced coma.

The lawsuit also accused Gordon of assault, battery, intentional infliction of emotional distress, unjust enrichment and conversion.

Fulton County District Attorney Paul Howard said he and his office are interested in reviewing the investigative file to determine whether any charges will be filed.

Bobbi Kristina was fabulously wealthy for a teenager, but her money was in a spendthrift trust, designed to keep creditors and predators from taking advantage of people who can't manage their money. Bobbi Kristina's grandmother, Cissy Houston, and aunt, Pat Houston, eventually took over control of the trust and then took Bobbi Kristina to court to protect the estate.

The size of Houston's estate is a privately held secret.

In May, a judge appointed Bobby Brown and Pat Houston as co-guardians of Bobbi Kristina, giving them joint responsibility in decisions related to her care and medical needs. Lawyer Bedelia Hargrove was appointed conservator to oversee Bobbi Kristina's assets, including her rights and legal claims.

By January 2014, the young couple who grew up as brother and sister were sharing a townhome and calling themselves husband and wife.

They posted images of their hands wearing wedding rings, with the caption "(hash)HappilyMarried. So (hash)InLove. If you didn't get it the first time that is." They got identical "WH" tattoos with flying doves on their wrists, and Gordon added a large portrait of Houston's face on his arm.

Experts: Time is an enemy in solving Bobbi Kristina's death

In the months since Bobbi Kristina Brown was found face-down in a bathtub at her townhome in suburban Atlanta until her death in hospice care Sunday, time has been working against forensic pathologists and police investigators now tasked with solving how and why she died.

Any drugs the 22-year-old daughter of Whitney Houston might have taken passed from her bloodstream long ago. Physical injuries would have been healing even as Brown remained largely unresponsive. If police overlooked any physical evidence at Brown's home after she was hospitalized Jan. 31, recovering it nearly six months later may be impossible.

The Fulton County Medical Examiner's Office issued a statement Monday saying it will perform an autopsy but cautioned that the passage of time may "complicate reconstruction of events surrounding her initial unresponsiveness." The agency didn't give a time frame for completing the autopsy but said any new lab tests required could take several weeks.

Dr. Michael Baden, former medical examiner for New York City, has performed more than 20,000 autopsies during a career spanning more than 45 years. He said the first obstacle for forensic pathologists in Brown's case will be a living body's ability to mend itself and erase medical evidence.

"Normally, when we do autopsies, we do them in people who freshly died. Things like toxicology and injuries are clear," said Baden, who helped investigate high-profile cases including the deaths of comedian John Belushi and civil rights worker Medgar Evers. "Because she was in the hospital for a long time, any drugs that may have been in the body will be gone after a few days. Injuries, if there were any injuries, would be changed by the length of time, the healing process."

That means Brown's hospital records will be as important, if not more so, to medical examiners investigating her death as physical evidence from her autopsy.

Baden said it's highly possible doctors screened Brown's blood for drugs as soon as she was admitted to the hospital. Her medical charts may also note any physical injuries observed by doctors who first treated her. The initial report by emergency medical technicians who first responded to Brown's home could also yield critical clues, he said.

Dr. Henry Lee, the forensic scientist whose famous cases include the death of child beauty queen JonBenet Ramsey, said the passage of time could make Brown's autopsy "much harder."

But he said valuable clues could possibly still be gained from examining her lungs and digestive system for lasting injuries or other abnormalities. And lab tests of any clothing she may have worn the day she was rushed to the hospital could turn up residue of drugs or other toxins.

"There's a lot of stuff you can do," Lee said.

The Roswell Police Department in suburban Atlanta is still investigating the events that led to Brown's death, authorities say. A police report described what happened in the townhome Brown shared with Nick Gordon as a drowning. However, no one has been charged in the case.

Gordon has said Bobbi Kristina, whose father was R&B singer Bobby Brown, wasn't breathing when she was found face-down in the tub - and that she lacked a pulse before first responders arrived. The scene was similar to how Houston was found dead in 2012 in what authorities later ruled an accidental drowning.

In the case of solving her daughter's death, much will depend on how thorough an investigation police launched when Brown was first hospitalized in January, said Ken Hodges, a former Georgia prosecutor who's now a defense attorney in Atlanta.

"The level of intensity that law enforcement gives things oftentimes is based on the severity of what they've got," said Hodges, who served 12 years as district attorney in Dougherty County. "In a homicide, it's often given to the more seasoned detectives. If this is perceived at the beginning as a drug overdose and not necessarily criminal, it may not be investigated at all or might be given to a less experience detective."

Any fingerprints, blood, hair samples or other physical evidence that police failed to collect six months ago would likely now have vanished, Hodges said. Or any evidence still at the scene months later - such as a fingerprint on a doorknob - would have far less value now because of potential comings and goings since.

Likewise, Hodges said, any witnesses who weren't interviewed by police at the time might have trouble remembering events correctly.

"They may have done it all six months ago," Hodges said. "But time is never a law enforcement officer's or a prosecutor's friend. It's always best to jump on a case early. Evidence disappears; memories fade. Things happen."