Police in Boston have surrounded a building in the suburb of Watertown as they hunt a teenager they suspect was involved in the bomb attack on the city marathon.
Local media is reporting that shots have been heard in the vicinity of an apartment block which had been cordoned off.
Earlier Tarmelan Tsarnaev was killed after a police chase and shootout in the Watertown area this morning. It is his 19-year-old brother Dzokhar who is still on the run.
His uncle Ruslan Tsarni lives near Washington DC. He says the family thought the boys had a bright future.
"From their mother I heard they have been straight-A students with goals to be in Harvard" he said.
"Just a big future - that's what I heard" he added.
Security officials in the US believe the two brothers are from a Russian region near Chechnya.
19-year-old Dzhokhar Tsarnaev (pictured above) is on the run while another man reported to be his 26-year-old brother Tamerlan was killed in a shootout earlier.
A major manhunt is underway in the city where public transport networks have been shut down and roughly 900,000 people have been told to stay indoors.
Police have now linked the marathon bombings on Monday with the fatal shooting of an officer at a university last night. The police officer was responding to a car jacking at MIT University when he was shot and killed.
There followed an over-night manhunt during which one of two bombing suspects identified by the FBI was shot dead by police.
People in Watertown near Boston are being told to stay indoors and keep away from windows.
The Boston Police Commissioner Ed Davis has described him as armed and dangerous amd they believe he is a "terrorist".
Footage of the MIT shootout was also captured on video.
Police are still searching for this man - Dzhokhar Tsarnaev
Seperatly the FBI last night released images and video footage of the two men. The footage shows them carrying backpacks on a crowded footpath near the finish line.
3 people were killed and over 170 injured in the blasts on Monday.
Speaking at a news conference FBI Special Agent in Charge in Boston Richard DesLauriers warned that they are considered "armed and dangerous".
He identified suspect No. 1 as a man wearing a dark baseball cap and sunglasses whle Suspect No. 2 who they are still searching for wore a white cap baseball cap backwards and was seen setting down his backpack on the ground.
The video was taken about 13 minutes before the two explosions.
