FBI investigates motive behind Boston terror attack.

By Stephanie Simon and Aaron Pressman

BOSTON, April 20 (Reuters) - With one of the suspects in the Boston Marathon bombings being treated in a hospital for serious injuries, investigators worked on Saturday to find the motive and determine if the Chechen brothers accused of the attack acted alone.

Dzhokhar Tsarnaev, 19, was captured late Friday night after a shootout with police that ended a nearly 24-hour manhunt. His brother, Tamerlan, 26, was killed Thursday in a shootout with police.

Tsarnaev was being treated at Beth Israel Deaconess Medical Center in Boston, said spokeswoman Kelly Lawman, who added that the FBI would provide further information on the young man's condition.

There was no clear information about when he would be officially charged.

Tsarnaev was hiding inside a boat parked in the backyard of a house in the suburb of Watertown, police said.

US President Barack Obama promised on Friday that the country would find out if the two Chechen brothers suspected of the Boston Marathon bombings received help, and urged Americans to refrain from judgment.

There are still doubts about the information released on Friday that the FBI questioned one of the suspects in 2011 and found no evidence that he was dangerous.

"We're going to determine what happened. We're going to investigate any association these terrorists may have, and we'll continue to do what we have to do to keep our people safe," Obama said regarding the two explosions that killed three people and injured 176.

Initial indications suggested the suspects acted alone, Watertown Police Chief Edward Deveau told CNN on Saturday.

"From what I know so far, these two acted both together and alone," he said.