* Whole of Boston virtually on lockdown
* Two suspects were brothers, officials say (Adds details of suspect's internet postings)
By Scott Malone and Tim McLaughlin
WATERTOWN, Mass., April 19 (Reuters) - Police killed one suspect in the Boston Marathon bombing in a shootout and mounted house house-to-house searches for a second man on Friday, with much of the city under virtual lock-down after a bloody night of shooting and explosions in the streets.
A national security official identified the hunted man as Dzhokar A. Tsarnaev, 19, and said the dead suspect was his brother, Tamerlan Tsarneav, 26. The brothers had been in the United States for several years, the official said.
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 M.I.T. and public schools were closed.
During the night a university 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 2, 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 1, 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 caused by bombs in pressure cookers placed inside backpacks left near the finish line of Monday's Boston Marathon. The blasts killed three people and wounded 176 in the worst attack on U.S. soil since the suicide hijacking attacks of Sept. 11, 2001.
A Russian language social networking site bearing Dzhokar Tsarneav's name paid tribute to Islamic websites and to those calling for Chechen independence. The author identified himself as a 2011 graduate of Cambridge Rindge and Latin School, a public school in Cambridge, Massachusetts.
He said he went to primary school in Makhachkala, capital of Dagestan, a province in Russia that borders on Chechnya, and listed his languages as English, Russian and Chechen.
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 with titles such as "Salamworld, my religion is Islam" and "There is no God by Allah, let that ring out in our hearts."
He also had links to pages calling for independence for Chechnya, a region of Russia that lost its bid for independence after two wars in the 1990s.
STEP BY STEP
About five hours after the FBI released the surveillance pictures showing the two men near the bombing site on Thursday, a university police officer was shot and killed on the campus of the Massachusetts Institute of Technology, the Middlesex County District Attorney said in a statement.
A short time later, police received reports of a carjacking by two men who kept their victim inside the car for about half an hour before releasing him, the statement said.
Police pursued that car to Watertown, where explosives were thrown from the car at police and shots were exchanged, the statement said. Suspected unexploded ordnance required a number of controlled explosions throughout the morning.
"During the exchange of the gunfire, we believe that one of the suspects was struck and ultimately taken into custody. A second suspect was able to flee from that car and there is an active search going on at this point in time," Colonel Timothy Alben, superintendent of the Massachusetts State Police, told a news conference.
The wounded suspect was taken to Beth Israel Deaconess Medical Center, where he died, said Dr. Richard Wolfe, chief of emergency medicine.
"This was a trauma arrest, multiple injuries, probably, we believe, a combination of blast, potentially gunshot wounds," Wolfe told a news conference. When asked how many gunshot wounds, he said: "Unable to count."
The blast injuries may have been caused by "an explosive device, possibly shrapnel, thermal injury. It was pretty much throughout the trunk. It was multiple wounds," he said.
TRANSPORTATION SHUT DOWN
Massachusetts Governor Deval Patrick suspended all public transportation service on the Boston-area subway, bus and rail system as a precaution, said Kurt Schwartz, director of the Massachusetts Emergency Management Agency.
Schwartz also asked people in the Boston-area communities of Newton, Waltham, Belmont, Cambridge and the Allston-Brighton to stay indoors and asked businesses in those areas to remain closed pending further notice.
(Additional reporting by Alex Dobuzinskis in Los Angeles and David Bailey in Minnesota; Writing by Daniel Trotta)