WATERTOWN, Mass.) — One suspect in this week's deadly Boston Marathon bombing was killed after a police shootout, and a second suspect, identified by officials as Dzhokhar Tsarnaev, remains at large and the subject of a manhunt, police said early Friday morning.

Tsarnaev, the man the FBI identified on Thursday only as Suspect 2 in photos related to the marathon bombing investigation, is on the loose and is armed and dangerous, police said.  He is the target of an "active search by tactical teams." 

"We believe this is a terrorist, we believe this is a man that's come here to kill people," Boston Police Commissioner Ed Davis said in a brief news conference Friday morning.
 

[ CLICK HERE TO SEE PHOTOS OF THE MANHUNT ]
 

The first suspect was killed after exchanging gunfire with police officers, during which multiple explosive devices were detonated, police said.  

Officials at Beth Israel Hospital reported they received one patient who later died, but would not confirm it was the first suspect.  That patient came in under guard and had suffered blast, shrapnel and so many gunshot wounds that caregivers were "unable to count."

Federal law enforcement sources told ABC News the suspects had been in the U.S. two years or longer and, because of the skill with which they engaged police, likely had para-military training.

One police officer was also injured and is in critical condition at a local hospital.  Authorities said the suspects, who have not been identified, were also responsible for the death of an MIT police officer Thursday night.

Police in Watertown, Mass., a suburb of Boston, blocked off at least one town street and multiple police cars, ambulances and fire trucks surrounded the scene after the firefight in a residential neighborhood.  Boston officials said on Twitter that all public transportation has been suspended amid the manhunt.

Massachusetts state police have tweeted guidance for Watertown residents to stay in their homes and not answer their doors unless it is for an identified police officer. 

Police are "going door by door, street by street, in and around Watertown. Police will be clearly identified. It is a fluid situation," according to the state police tweet.

Police said the violent ordeal began around 10:30 p.m. Thursday, when a 7-Eleven convenience store was robbed in Cambridge, Mass., shortly before the MIT officer was shot.  The suspects then hijacked a Mercedes and drove to Watertown while being pursued by police. 

The chase ended in a firefight that left one suspect dead, while the other managed to slip away, police said.  Federal law enforcement sources told ABC News the suspects admitted their role in the Boston Marathon bombing to the carjack victim.

Monday's bombing near the finish line of the Boston Marathon killed three people and injured more than 170 others.  The FBI released images Thursday evening of the two suspects in the case.

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