Worries reverberated across the Atlantic, where an already robust security operation was being beefed up for Wednesday’s ceremonial funeral for former Prime Minister Margaret Thatcher.

A person who spoke on the condition of anonymity to protect the family’s privacy confirmed the death of an 8-year-old boy.

Google stepped in to help family and friends of runners find their loved ones, setting up a site called Google Person Finder.

The FBI said it had set up a phone line for members of the public to call with information about the explosions.

Officials said there was no report of a specific threat to New York, but city residents were urged to be vigilant as police took both highly visible and unseen precautions.

Whoever was responsible for the bombs made sure it was not only horrific but well-documented. The Boston Marathon finish line is a place certain to be filled with cameras held by professionals and amateurs alike.

The B.A.A. has managed the Boston Marathon since the late 19th century.

Two men associated with Run for the Fallen in Maine were across the street from the first explosion.