Hazmat biohazard team at Boston Marathon near the finish line 03

This is all over the news - clipping from NY Times: A series of bombs exploded near the finish line at the Boston Marathon on Monday, leaving two people dead and more than two dozen injured, according to the Boston Police Department. The explosions at the Boston Marathon went off more than four hours after the start of the men’s race, which meant that there were still several thousand runners yet to finish the race. Readers’ Comments 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 Associated Press, citing an intelligence official, said that two other devices were found at the marathon and were being dismantled.

People were being cleared from an area near the Copley Plaza Hotel because a package was found on a footbridge nearby.

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.

The Boston police confirmed they were looking into the explosions, but made no further comment. A 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 threatened 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.

Eyewitnesses said the explosions were about 15 seconds apart. A few runners stopped and turned around after they heard the blasts. Some runners were seen crying and some said they saw limbs on the ground. Runners were directed to the runners’ meeting area and hotels several blocks away.

Bruce Mendelsohn, who was at a party 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, while also examining the entire route just before the race. They would also place snipers on rooftops, with helicopters overhead. Analytic cameras in the city would also 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 running’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. The Police Department has sent officials to Boston as well.

My deepest condolences go to the family and friends of the wounded and killed here.