Washington, Apr 20 (IANS): US President Barack Obama has vowed to find out the motives behind Boston Marathon bombings that killed three people and left about 180 injured in the first major terror attack on American soil after 9/11.

"Obviously, tonight, there are still many unanswered questions," Obama said speaking from White House briefing room after 10 p.m. Friday, just over an hour after the remaining suspect, 19-year-old Dzhokhar Tsarnaev was captured after a massive manhunt.

Among the "many unanswered questions," Obama said, were why these men, "who grew up and studied here as part of our communities" resorted to such violence.

He asked, "How did they plan and carry out these attacks and did they receive any help?"

Obama said he had instructed the FBI and Department of Homeland Security to continue to help in the investigation of whether Dzhokhar and his older brother Tamerlan, 26, who was killed in an overnight police shootout, had help.

"We will determine what happened. We will investigate any associations these terrorists may have had," he said.

The capture of a second suspect in the Boston Marathon bombings "closed an important chapter in this tragedy," he said. But "The families of those killed so senselessly deserve answers," he said.

Obama said he had ordered the "full resources of the federal government" to help in the investigation that he said wouldn't have been possible without close coordination among federal, state and local authorities.

Noting that in tragedies such as this, there is a tendency of the public to "latch on to any bit of information." Obama cautioned against this, saying that when public safety is at risk, "the stakes are high, so it's important that we do this right."

"That's why we take care not to rush to judgment - not about the motivations of these individuals, certainly not about entire groups of people," he said in an apparent reference to the suspects reported backgrounds in Chechnya, the disputed Muslim region in Russia.

Obama said no one should be judged solely on their background and all defendants are entitled to a fair trial.

"All in all," he said, "this has been a tough week. But we have seen the character of our country once more."

"We have the courage, resilience, and spirit to overcome these challenges and to forward as one nation, under God, indivisible, with liberty and justice for all."