By Steve Holland, Matt Spetalnick and Gabriel Debenedetti
WASHINGTON (Reuters) - President Barack Obama pledged on Friday that the United States will find out whether the two ethnic Chechen brothers suspected of carrying out the Boston Marathon bombings received help, and he pleaded for Americans not to rush to judgment.
Obama appeared in the White House briefing room after police arrested the lone surviving suspect in the Boston suburb of Watertown, ending a dramatic manhunt. The other suspect was killed in a shootout overnight with police.
The U.S. leader watched the fast-paced developments on television in the White House residence, then returned to the Oval Office where he was briefed by FBI Director Robert Mueller. Relief swept the White House at the news of Dzhokhar Tsarnaev's arrest but there was no sign of a celebration.
"Obviously tonight there are still many unanswered questions. Among them: why did young men who grew up and studied here as part of our communities and country resort to such violence? How did they plan and carry out these attacks, and did they receive any help?" Obama said.
The successful conclusion of the manhunt allowed Obama to tout a major law enforcement achievement in response to the worst attack on U.S. soil since the September 11, 2001, attacks.
Questions remain, however, over the FBI's disclosure on Friday that it had interviewed one of the suspects in 2011 and found no evidence that he posed a security risk.
The president, looking somber and gripping the podium, said Americans are in debt to the people of Boston and Massachusetts for their resilience in responding to the twin blasts that killed three people and injured 176 others on Monday and enduring a wrenching week.
"We will determine what happened. We will investigate any association that these terrorists may have had and will continue to do whatever we have to do to keep our people safe," Obama said.
In urging Americans to show tolerance, Obama may have been referring to the surviving suspect who is known to have posted links to Islamic websites calling for Chechen independence.
Obama appealed for Americans to avoid a rush to judgment, saying people should stay true to the "unity and diversity that makes us strong."
"That's why we have courts. 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 ... We welcome people from all around the world, people of every faith, every ethnicity," he said.
Obama spoke earlier in the day with Russian President Vladimir Putin about the Boston bombings and the White House said he praised U.S.-Russian counter-terrorism cooperation including after Monday's attack.
The end of the Boston manhunt capped an emotional and difficult week for Obama.
His legislation to tighten background checks on gun buyers, a response to the December massacre of 20 children and six adults at a school in Connecticut, went down in a bitter defeat in the U.S. Senate, prompting Obama to angrily denounce it as a "shameful day" in Washington.
And with the nation already on edge, authorities intercepted letters laced with ricin, a highly lethal poison, that were sent to Obama and Republican U.S. Senator Roger Wicker. Authorities have arrested a Mississippi man in the case.
Obama also attended a wrenching inter-faith service for the victims of the bombings in Boston on Thursday.
"All in all, this has been a tough week," he said. "But we've seen the character of our country once more."
(Editing by Paul Simao)