Two US officials told the Associated Press Monday that preliminary evidence suggests the Boston suspects were motivated by religion.
However, it appears they were not tied to any Islamic fundamentalist groups, and worked alone.
The U.S. officials spoke Monday on condition of anonymity because they were not authorized to publicly discuss the investigation.
Some of the evidence may have come from the mosque the two suspects sometimes attended.
The Islamic Society of Boston Cultural Center says in a statement released Monday that Tamerlan Tsarnaev argued with a preacher during a service at its Cambridge mosque in November about U.S. holidays.
The center says he returned to services and in January called an elder a "hypocrite" for praising Martin Luther King Jr.
The center says congregants shouted at him, telling him to leave. The mosque says there were no further incidents after that, even as he continued attending prayers and services.
Surviving suspect formally charged
The U.S. Attorney General says the surviving suspect in the Boston Marathon bombings has been charged with conspiring to use a weapon of mass destruction against persons and property resulting in death.
We are also learning more about the possible motive for the bombings.
In a statement Monday, U.S. Attorney General Eric Holder detailed the charge against 19-year-old Dzhokhar Tsarnaev.
The charge carries a possible death sentence.
Tsarnaev made his initial court appearance in his room in Beth Israel hospital. He is listed in serious but stable condition.
Officials say Tsarnaev and his older brother set off the twin explosions at Monday's marathon, killing three people and injuring more than 180 others.
Officials said it was not an arraignment.
The White House said on Monday that the Obama administration will not treat Tsarnaev as an "enemy combatant."
Tsarnaev also is likely to face state charges in connection with the fatal shooting of MIT police officer Sean Collier in Cambridge, said Stephanie Guyotte, a spokeswoman for the Middlesex District Attorney's office.
Many Boston residents headed back to workplaces and schools for the first time since a dramatic week came to an even more dramatic end. Traffic was heavy on major arteries into the city Monday morning, and nervous parents dropped their children off at schools, some for the first time since the attacks.
Authorities on Friday had made the unprecedented request that residents stay at home during the manhunt for suspect Dzhokhar Tsarnaev. He was discovered that evening hiding in a boat covered by a tarp in suburban Watertown.
Tsarnaev is a naturalized U.S. citizen. Carney says that under U.S. law U.S. citizens cannot be tried in military commissions. White House Spokesman Jay Carney says that since Sept. 11, 2001, the federal court system has been used to convict and incarcerate hundreds of terrorists.
Tsarnaev and his older brother and suspected co-conspirator, Tamerlan Tsarnaev, were born in southern Russia.