WATERTOWN, Massachusetts (AFP) - United States (US) authorities said on Friday they would carry out a "controlled explosion" before a search of a house that may have been used by the two Boston Marathon bomb suspects.
They also maintained a "stay indoors" order for hundreds of thousands of people in the region so the hunt by thousands of police can be extended and new leads pursued, officials said.
One suspect, Tamerlan Tsarnaev, 26, was killed in a shootout with police early Friday and his brother Dzhokhar Tsarnaev, 19, is now on-the-run.
The ethnic Chechens who had been living in the US for several years have been closely linked to the bombing Monday in which three people were killed and 180 injured.
State police colonel Timothy Alben told reporters the precautionary explosion would be carried out for the safety of law enforcement officials "before they proceed with a search" of a house.
Authorities ordered a lockdown across much of the Boston region and halted all public transport, including the main Boston-to-New York railway, to intensify the hunt.
The pursuit is now focused on Watertown and Cambridge, in the Boston suburbs, where the brothers lived.
"We are progressing through this neighbourhood, going door-to-door, street-to-street. We are well over 60 or 70 percent of what we want to cover," the state police chief said.
"We've got several other new leads that just developed within the last few minutes and we're working on that," Mr Alben added.
Massachusetts governor Deval Patrick said "the stay indoor request" would continue.
"There are continuing developments in the investigation," Patrick said, adding that it remained important for people to "keep the doors locked" because of the danger posed by the remaining suspect.
The Amtrak train company said its Acela Express and Northeast Regional service remains suspended indefinitely in the Boston area, following requests from authorities.
Trains from the south are halting in New York.