Obama Boston bombings arrest
Obama addresses the nation (NBC)

President Barack Obama admitted that many questions remain following the capture of the second suspect in the Boston bombings on Friday 19 April.

The president said the arrest of 19-year-old Dzhokhar Tsarnaev "closed an important chapter in this tragedy", but added: "The families of those killed so senselessly deserve answers."

Intelligence agencies, the FBI and the Department of Homeland Security were continuing to investigate possible motives for the bombings and subsequent shooting spree which culminated in Tsarnaev's arrest aboard a boat in the Boston suburb of Watertown.

His brother Tamerlan Tsarnaev, 26, was killed in a shoot-out with police on Thursday.

"After a vicious attack on their city, Bostonians responded with resolve and determination. They did their part as citizens and partners in this investigation," the president said in a speech from the White House soon after Dzhokhar Tsarnaev's arrest.

Calling the two brothers "terrorists", Obama added: "Boston police and state police and local police across the commonwealth of Massachusetts responded with professionalism and bravery over five long days.

"The wounded, some of whom now have to learn how to stand, walk and live again deserve answers. And so I've instructed the FBI and the Department of Homeland Security and our intelligence community to continue to deploy all the necessary resources to support the investigation, to collect intelligence and to protect our citizens.

"All in all it's been a tough week, but we've seen the character of our country once more.

"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 our country resort to such violence? How did they plan and carry out these attacks and did they receive any help?

"The families of those killed so senselessly deserve answers, the wounded, some of whom now have to learn to stand, walk and live again deserve answers."

Three people were killed and more than 180 injured in Monday's twin marathon explosions. A policeman at the Massachusetts Institute of Technology was shot dead and another severely wounded during the subsequent search for the suspects, identified as ethnic Chechens from southern Russia who had lived in the US for a decade.

"I'm confident that we have the courage and the resilience and the sprit to overcome these challenges and to go forward," Obama said.

Obama's comments came after he spoke to Russian President Vladimir Putin, thanking him for his cooperation on counter-terrorism "including in the wake of the Boston attack".

Obama warned against rushing to premature judgments as to the motive for the attacks, and cautioned against ascribing an ethnic agenda to the bombings.

"That's why we have investigations. That's why we relentlessly gather the facts. That's why we have courts," he said.

The president also turned to the explosion at the fertiliser plant in West, Texas, on Wednesday, in which at least 14 people were killed, 200 injured and another 60 are still missing.

Describing the community as "tightknit", he pledged resources for rescue and reconstruction efforts, saying: "I want them to know that they are not forgotten."

A man was charged with threatening the president's life on Thursday after letters addressed to Obama and a US senator, were found to contain traces of the poison ricin.