BOSTON: One of the Boston bombing suspects reportedly died on Friday after a shootout with police, as explosions rang out near the tense US city and officers stayed on the hunt for a second suspect.

The manhunt began after a campus police officer was killed at the elite Massachusetts Institute of Technology, the Boston Globe reported, days after the city's historic marathon came under attack.

The first suspect was caught by police in the Watertown district north of MIT, and US media said he later died of his wounds.

Describing the shootout, local resident Adam Brown told AFP: “There were blasts, it could have been grenades.”

MIT, one of the world's top universities, is situated in Cambridge, Massachusetts, just across the Charles River from Boston where three people were killed and 180 injured in a double bomb attack on the marathon on Monday.

Authorities made no comment on a link between the manhunt and the bombing at the marathon finishing line, the worst terror attack in the United States since the September 11 atrocities in 2001. There has been no claim of responsibility.

But the dramatic developments came shortly after the FBI released pictures and video of the two men suspected of planting the bombs, appealing for help to identify the pair who were carrying large backpacks.

Both appeared to be young men, one dressed in a white baseball cap and the other in a black cap. The FBI gave no details of their identities or origin, naming them only as Suspect One and Suspect Two.

Authorities said that a second police officer was wounded in the dramatic nighttime operation which saw heavily armed police swoop on the Watertown district and cordon off a large area of the university and surrounding streets.

Media reports said the events cascaded with a robbery at a gas station followed by the killing of the campus officer and then a car chase to Watertown.

The NBC-WJAR channel showed film of a man lying on the ground in a street in the town and surrounded by police.

The university warned students to stay indoors as police with rifles patrolled the streets and search helicopters flew above the campus.

“The shooter remains at large; police continue to search the campus. Please remain indoors until further notice,” said an emergency alert issued by the university on its website.

The MIT campus police officer suffered “multiple gunshot wounds” when he responded to a disturbance, said Middlesex District Attorney Michael Pelgro in a statement.

The officer “was transported to the hospital and pronounced deceased,” said the statement, which added that there were no other victims.

Two bombs were placed around the marathon finish line on Monday, spraying nails, ball bearings and other metal fragments into massed spectators, many of whom suffered horrific injuries.

The men are seen in the video walking calmly, one a few paces behind the other, weaving between crowds on Boston's Boylston Street where the race finished.

President Barack Obama vowed to the people of Boston Thursday that the “evil” bombers would be brought to justice.

At a special service at Boston's Cathedral of the Holy Cross, Obama vowed: “Yes, we will find you, and yes, you will face justice.”

We will find you, we will hold you accountable,” he told a congregation of 2,000, including relatives of the dead, survivors of the blasts, rescuers and city leaders.

“If they sought to intimidate us, to terrorize us,” Obama said, then “it should be pretty clear by now that they picked the wrong city to do it.”

Boston has held emotional tributes to the dead — eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and Krystle Campbell, a restaurant manager.

Obama paid tribute to all three at the service.

More than 100 of the wounded have left Boston hospitals and fewer than 10 of those still in hospital remain in critical condition. Some will require new operations, doctors said.