WASHINGTON (REUTERS) - President Barack Obama pledged on Friday that the United States (US) will find out whether the two brothers suspected of carrying out the Boston Marathon bombings received help.

Ending a stressful week, Mr Obama appeared in the White House briefing room after police arrested the lone surviving suspect in the Boston suburb of Watertown, ending the manhunt. The other suspect was killed in a shootout overnight with police.

"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?" Mr Obama said.

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.

Mr Obama also expressed his condolences to the people of West, Texas, after an explosion on Wednesday at a fertiliser plant killed at least 14 people. He vowed federal resources to help the people there.