BOSTON/WASHINGTON - A suspect of the deadly Boston Marathon bombings has been nabbed, authorities announced Friday night, wrapping up a day-long manhunt.

US President Barack Obama lauded the law enforcement efforts in detaining 19-year-old Dzhokhar Tsarnaev, but also noted that many questions remain to be answered.

FBI officers work in front of the house at 67 Franklin St. after the capture of Dzhokhar Tsarnaev, the surviving suspect in the Boston Marathon bombings, in Watertown, Massachusetts April 19, 2013. [Photo/Agencies]

At a news conference before a cheering crowd, Col. Timothy Alben, superintendent of Massachusetts State Police, said: "We are eternally grateful for the outcome tonight. We have a suspect in custody."

The arrest came after a massive manhunt that saw the city of Boston locked down. Just after the authorities announced that they were lifting the lockdown, a resident in Watertown, a suburb of Boston where most of the operation took place, alerted police of blood on a boat in the backyard of his house.

"There was a call that came in to the Watertown Police," Boston Police Commissioner Ed Davis said. "He saw blood on the boat in the backyard. He opened the tarp and saw a man covered with blood. He retreated and called us."

Police officers and Federal Bureau of Investigation (FBI) agents rushed to the scene and found Tsarnaev, a suspected bomber in Monday's deadly blasts at the Boston Marathon finish line, he added.

The officers then "set up a perimeter around the boat and, over the course of the next hour or so, exchanged gunfire" with the suspect, Davis said.

A hostage negotiation team from the FBI eventually "removed the suspect", who was in "serious condition", he said, without providing further details.

Davis also said the FBI had earlier in the day taken three more people into custody in connection to the bombings.

US District Attorney Carmen Ortiz applauded law enforcement for their work, adding that investigation was ongoing and her office would crack the motives behind the bombings and file formal charges.

Despite the detainment, "there are still many unanswered questions," such as what motivated the bombers and who helped them, Obama said in a televised statement from the White House shortly after the detainment.

"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?" Obama added.

The president also mourned the victims in the bombings and the following police operation, and praised the law enforcement and local residents in greater Boston area.

The American people, he stressed, refused to be terrorized by such a "heinous act."

The manhunt for Tsarnaev started late Thursday when his elder brother, Tamerlan Tsarnaev, also a bombing suspect, was killed in an overnight shootout with police.

An army of law enforcement officers swarmed Cambridge and Watertown in Boston suburbs, trying to find him.

The brothers, whose photos were released by the FBI on Thursday, were identified as Suspects No 1 and No 2 in the marathon bombings, which killed three and injured more than 170.

During the manhunt operation, a campus police officer was killed at Massachusetts Institute of Technology, and another was severely wounded.