Police have "sealed" Boston and stand alert at key locations in the city as they spare no efforts to capture the remaining suspect in the Boston Marathon bombing.

A Xinhua reporter said that fewer vehicles could be seen on the streets of Boston early Friday. Residents throughout the Boston closed their doors tightly, and the city was wrapped in a nervous atmosphere.

Police have advised local residents to stay indoors and not to open their doors to strangers as the fugitive, 19-year-old Dzhokhar Tsarnaev, likely was armed and may possess explosives.

Dzhokhar and his brother, Tamerlan Tsarnaev, reportedly were of Chechen origin and had been in the United States for years after immigrating from Russia's Chechnya region@ Dzhokhar won a scholarship in 2011 while enrolled at the Cambridge Rindge and Latin School, a prestigious public high school.

Tamerlan, 26, was killed during a police chase. A website said Tamerlan was studying engineering at Bunker Hill Community College and he spent years in Kazakhstan before coming to the United States.

Chechnyan officials have said the Boston bombing suspects have no ties with Chechnya.

The Tsarnaev brothers grew up in a Russian region near Chechnya but have had little to do with the area for years, said a spokesman for Chechnyan leader Ramzan Kadyrov.

Spokesman Alvi Karimov told reporters that the brothers lost connection with the Chechnya republic many years ago, noting "The people suspected of committing crimes in Boston have nothing to do with Chechnya."

The Tsarnayev family left Chechnya for Kazakhstan and moved to the United States from there, Karimov said.

The two brothers were born and raised in the Russian city of Makhachkala, the capital of Dagestan republic, said Temirmagomed Davudov, their former school teacher.

"The two brothers and their two sisters used to study in our school for one year," said Davudov, director of Makhachkala's No. 1 public school.

He said the brothers left Dagestan in 2002 while they still were minor children.

U.S. President Barack Obama was being briefed Friday morning with his national security team on the Boston bombing investigation, a White House official said.

Obama, joined by Vice President Joe Biden, convened the briefing at the White House with senior officials in his national security team, including Attorney General Eric Holder, FBI Director Robert Mueller, Chief of Staff Denis McDonough, and National Security Advisor Tom Donilon.

The briefing came after an armed showdown and confrontation between police and the Tsarnaev brothers occurred in the Boston area overnight.

In another development, a gray Honda CRV with Massachusetts license plates that was believed to be occupied by a possible suspect linked to the bombing, has been located, police said.

A shooting occurred at approximately 10:20 p.m. Thursday local time near the MIT campus and a campus police officer was killed. Police believed the incident was linked with the two suspects.

The Middlesex County district attorney said in a statement Friday that the suspected bombers first tried to rob a convenience story in Cambridge, across the river from Boston. They then went to the MIT campus where they shot at the police officer several times.

Afterwards, the two carjacked a Mercedes Benz and eventually let the driver out at a gas station in Cambridge. The search for the vehicle led to a chase that ended in Watertown.

Police said the suspects threw explosive devices from their car and exchanged gunfire with officers. A transit police officer was seriously injured during the chase, authorities said.

In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 a.m. Friday local time. Dozens of police officers and FBI agents were in the neighborhood and a helicopter circled overhead.

Police urged people in Watertown, Cambridge and other western suburbs, as well as some western neighborhoods of Boston, to stay indoors as the search for Dzhokhar Tsarnaev continued.

Authorities suspended public transportation, including buses, subways, trolleys, commuter trains and boats, in the Boston region during the search.

The violence and manhunt began only a few hours after the FBI released photos and videos of the two suspected bombers, who were seen carrying backpacks as they walked near the marathon finish line.

Monday's bombings at one of the world's most storied races killed three people and wounded more than 180 others. Authorities released the images of the suspects to an effort to have the public help locate the suspects.