One of the Boston marathon bombing suspects was killed in a shootout early Friday as police conducted a house-to-house search for the second, with the entire city placed on lockdown.

A national security official identified the hunted man as Dzhokar A. Tsarnaev, 19, and said the dead suspect was his brother, Tamerlan Tsarnaev, 26.

The brothers of Chechen origin had been in the US for several years, Reuters quoted the official as saying.

Authorities cordoned off a section of the suburb of Watertown and told residents not to leave their homes or answer the door as officers in combat uniforms carrying rifles scoured a 20-block area for the missing man, who was described as armed and dangerous.

Boston came to a virtual standstill after authorities urged everyone to stay at home. Public transportation throughout the metropolitan area was suspended and air space was restricted. Universities including Harvard and Massachusetts Institute of Technology (MIT) and public schools were closed.

During the night, an MIT police officer was killed, a transit police officer was wounded, and the suspects carjacked a vehicle before leading police on a chase that resulted in the suspect being shot dead.

Police were searching for the younger Tsarnaev, previously known only as Suspect Two, who was shown wearing a white cap in surveillance pictures taken shortly before Monday's explosions and released by the FBI on Thursday.

"We believe this to be a terrorist," said Boston Police Commissioner Ed Davis of the suspect still at large. "We believe this to be a man who has come here to kill people. We need to get him in custody."

The older brother, previously known as Suspect One, who was seen wearing a dark cap and sunglasses in the FBI images, was pronounced dead.

The FBI on Thursday identified the men as suspects in the twin blasts believed to have been caused by bombs in pressure cookers placed inside backpacks left near the finish line of Monday's Boston marathon. The blasts killed three people, including a Chinese graduate student at Boston University, and wounded 176 in the worst attack on US soil since the suicide hijacking attacks of September 11, 2001.

A Russian language social networking site bearing Dzhokar Tsarnaev's name paid tribute to Islamic websites and to those calling for Chechen independence. The author said he went to primary school in Makhachkala, capital of Dagestan, a province in Russia.

His "world view" was listed as "Islam" and his "personal priority" as "career and money."

He posted links to videos of fighters in the Syrian civil war and to Islamic Web pages. He also had links to pages calling for independence for Chechnya from Russia.

A man identified as father of the brothers Friday told the Interfax news agency from the North Caucasus Russian city of Makhachkala that his sons are "innocent of any crime."

"In my opinion, my children were set up by the secret services because they are practicing Muslims," Anzor Tsarnaev said.

Zhao Huasheng, director of the Center for Russian and Central Asian Studies at Fudan University, Friday told the Global Times it is still too early to determine the bombers' motives simply from their origin, and Chechen militants, who carry out terrorist attacks mainly in Central Asia and the Caucasus region, had never launched such attacks in US territory before.

"There are connections between Chechen militants and Islamist extremists for sure. They go across borders to join battles and carry out terrorist acts," said Zhao.

China's foreign ministry spokeswoman Hua Chunying said on Friday the Chinese side has thanked the US for aiding the relatives of two Chinese victims of the Boston Marathon bombing, Xinhua reported.

The parents of the deceased Chinese student left for the US on Friday.

Hua told a regular press briefing that China hopes the US will conduct a thorough investigation as soon as possible and bring those responsible for the bombing to justice.

Boston University on Thursday held a memorial service to mourn the Chinese student.

The university has set up a scholarship fund in her name and has already received contributions totaling $560,000, according to the fund's online campaign page.

US President Barack Obama grieved for her Thursday at an interfaith service honoring the victims of the bombings.

"And in the heartache of her family and friends on both sides of the great ocean, we're reminded of the humanity that we all share," Obama said.

Zhou Danling, another Boston University student, has been hospitalized and is in stable condition, according to Chinese consulate officials in New York.