Those remnants included: parts of one or two kitchen pressure cookers that had evidently been packed with nails, ball bearings and black powder and used as explosive devices; the torn remains of a dark nylon backpack or duffel bag in which one of the bombs had been hidden; and a circuit board, wires and other parts from timing devices. Investigators hoped to track the items back to where they were sold and compile a list of names or descriptions of the buyers.

A piece of the lid of one of the pressure cookers was found on a rooftop near the blast, a law enforcement official said on Wednesday — giving a sense of the tremendous force of the explosion.

The possible break in the case came as investigators scrutinized scores of videos and photographs from surveillance cameras from nearby businesses, as well as from marathon spectators’ smartphones and television crews that were filming the Boston Marathon when the deadly blasts went off. So far, no one has taken responsibility for the explosions.

As the investigation went into a third day, there were signs of jitters around the nation, which was on high alert. New York City officials said there had been an increase in reports of suspicious packages. In Oklahoma City, the scene of a devastating bombing in 1995, City Hall was briefly evacuated Wednesday morning as the authorities examined a stolen rental truck that was parked outside. (There was no bomb, officials there said.)

In Washington, parts of two Senate office buildings were shut down as officials investigated reports of suspicious letters or packages, and the Secret Service said that a letter addressed to President Obama contained a suspicious substance. It was intercepted at a screening facility outside the White House, and federal agents arrested a suspect on Wednesday evening.

The police in New York received 143 reports of suspicious packages between Monday afternoon, just after the Boston explosions, and midnight on Tuesday. This was an increase of more than 300 percent over a similar time period last year, said Raymond W. Kelly, the police commissioner.

And in Boston, the John Joseph Moakley United States Court House was evacuated in the afternoon as officials called out “code red” and bomb-sniffing dogs were sent inside.