Boston Marathon and JFK Library Blasts

A third explosion was heard just before 4 p.m., about an hour after the first two blasts. The police were apparently aware of that device before the explosion occurred.

The blasts took place about four hours after the start of the men's race, which meant that there were still several thousand runners yet to finish the race. One senior counterterrorism official said it was too soon to tell whether the explosions were related to terrorism.

The Boston police confirmed they were looking into the explosions, but made no further comment. Another senior United States government official said that the Boston police and the F.B.I. said they had received no reports in recent days about a threat of an attack on the marathon and that there was no warning on Monday.

Several news outlets reported that a loud explosion was heard on the north side of Boylston Street, near a photo bridge that marks the finish line. Another explosion was heard several seconds later.

The Associated Press reported that authorities were helping injured runners leave the scene and that bloodied spectators were being carried to a medical tent that was being used for runners.

Bruce Mendelsohn, who works in a building near the explosion, said on Twitter that he saw blood on the sidewalks and about a dozen casualties.

Authorities in large cities are typically on the highest level of alert for events like a marathon, said Anthony Roman, a security expert.

"It is quite the counterterrorism effort," said Mr. Roman, who runs Roman & Associates, a New York firm.

For major events in New York and other large cities, Mr. Roman said the police would typically weld manhole covers shut and examine the entire route just before the race. They would also place snipers on rooftops, with helicopters overhead, and analytic cameras in the city would be used, he said.

"They have all the analytic cameras in the city focusing on the race with their advanced software network, reading license plates," Mr. Roman said.

The Boston Marathon is one of track's most storied events, established in 1897 and one of the six World Marathon Majors. The event typically attracts an estimated 500,000 spectators and requires certain qualifying times for runners to compete.

The course winds throughout downtown Boston as well as several outlying cities, including Ashland, Framingham, Natick, Wellesley and Newton.

Unlike many sporting events that take place in closed arenas, marathons are known and heralded for their sprawl, allowing throngs of spectators to line the 26.2-mile course.

Within minutes of the explosions on Monday, social media and cable networks projected the images of gray smoke on Boylston Street, with emergency crews on the scene.

After the explosions, a spokesman for the New York Police Department said security was being increased at hotels and other prominent locations in New York.