One suspect in this week's deadly Boston Marathon bombing died today 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 this morning.
Law enforcement sources said the two suspects, deemed Suspect 1 and 2 Thursday in the marathon bomb investigation, are brothers of Chechen ethnicity. Tsarnaev, 19, who has now been on the run from authorities for more than 10 hours, is believed to be armed with an assault-style rifle and an assortment of other weapons, including bombs, a senior federal law enforcement official said.
Boston is on lockdown and a "massive manhunt" is underway, Massachusetts Gov. Deval Patrick said.
Boston Police Commissioner Ed Davis said early this morning, "We believe this is a terrorist, we believe this is a man that's come here to kill people."
At an early afternoon press conference, police revealed they had discovered an explosive device in a house in Cambridge that will be purposefully detonated later today. Massachusetts State Police Superintendent Col. Timothy Alben said authorities are going home-to-home in search of the suspect and are chasing "several new leads."
Tsarnaev's brother, Tamerlan, 26, was killed after he and his brother exchanged gunfire with police officers early today, police said. Multiple explosive devices were detonated during the firefight.
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" them.
One police officer, 33-year-old Richard Donohue Jr., was also injured in the firefight and is in critical condition at a hospital. Authorities said the suspects were also responsible for the earlier death of MIT police officer Sean Collier, 26, Thursday. Collier was sitting in his car when he was suddenly ambushed by the gunmen, two senior law enforcement officials said.
Citing the skill with which the suspects engaged police, federal law enforcement sources said they believe the men likely had paramilitary training.
Tsarnaev's father told ABC News he was worried about his sons when he heard about the bombing, but never imagined they could be involved.
Anzor Tsarnaev directed a message to his boys, saying, "Give up. Give up. You have a bright future ahead of you."
He added, however, that if police killed his son, "then all hell will break loose."
A neighbor of Tsarnaev's said he has been in the country at least since he was 7. A school friend of Tsarnaev's, Sierra Schwartz, told "Good Morning America" the young man "never seemed suspicious or weird or anything."
Boston officials said that all public transportation and taxi service has been suspended amid the manhunt, and Mayor Patrick issued a city-wide "shelter in place" order as the investigation unfolds.
"Please understand we have an armed and dangerous person(s) still at large and police actively pursuing every lead in this active emergency event," the order said.
Police said the violent ordeal began around 10:30 p.m. Thursday evening when a 7-Eleven convenience store was robbed in Cambridge, Mass., shortly before the MIT officer was shot. The brothers then hijacked a Mercedes and drove to Watertown, Mass. Police pursued the duo into a residential area of Watertown, a suburb of Boston.
The chase ended in a firefight that left Tsarnaev's brother dead, while he managed to slip away as police surrounded the area with multiple police cars, ambulances and fire trucks. Federal law enforcement sources told ABC News the suspects admitted their role in the Boston Marathon bombing to the carjacking victim.
A representative for 7-Eleven told ABC News later today the police were apparently mistaken, because their store was not the one robbed.
Earlier in the day, Massachusetts state police tweeted guidance for Watertown residents to stay in their residences and not answer their doors unless it is for an identified police officer.
Police were "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.
The Monday bombing near the finish line of the Boston Marathon killed three people and injured more than 170 more. The FBI released images Thursday evening of the two suspects in the case.
ABC News' Aaron Katersky contributed to this report.