WASHINGTON, D.C. – The number of children still waiting to be reunited with their families after more than two years since they were separated at the Mexican border is believed to be higher than previously disclosed, according to a new report. 

In an email to Justice Department attorneys obtained by NBC News, Steven Herzog, the lawyer leading efforts to reunite families, said the number of children whose parents have not been located is believed to be 666. Last month, court-appointed lawyers told a federal judge they had been unable to find the parents of 545 kids who were separated before and during the Trump administration’s now-abandoned “zero tolerance” immigration policy between July 1, 2017, and June 26, 2018.

Children from that period are difficult to find because the government had inadequate tracking systems. Volunteers have searched for them and their parents by going door-to-door in Guatemala and Honduras.

In the email, Herzog explained that the higher number is because the newer group includes children for which the government did not provide any phone numbers, NBC News reported.

“We would appreciate the government providing any available updated contact information, or other information that may be helpful in establishing contact for all 666 of these parents,” Herzog wrote.

Nearly 20% of those kids were under the age of 5 when they were separated, NBC reported, citing a source familiar with the data. 

More than 2,700 children were separated from their parents in June 2018 when U.S. District Judge Dana Sabraw ordered an end to the practice of criminally prosecuting every adult who entered the country illegally from Mexico. The administration sparked an international outcry when parents couldn’t find their children.

While those families were reunited under court order, authorities later discovered that up to 1,556 children were separated under the policy going back to the summer of 2017, including hundreds during an initial run at family separation in El Paso, Texas, from July to November 2017 that was not publicly disclosed at the time.

In his campaign ads, President-elect Joe Biden vowed to create a government task force to reunite all migrant families that were separated.

Asked about the inability to find the parents last month, White House spokesman Brian Morgenstern told reporters: “Many of them have declined to accept their children back. … It's not for lack of effort on the administration’s part."

At the final presidential debate, Trump defended his administration’s child-separation policy.

“Children are brought here by ‘coyotes’ [human smugglers] and lots of bad people, cartels, and they’re brought here and it’s easy to use them to get into our country,” Trump said. “We now have a stronger border than we’ve ever had.

"They are so well taken care of," Trump added of the children. "They’re in facilities that were so clean.”

The Associated Press contributed to this report.