Law-enforcement officials say they have no doubt that the explosions near the finish line of the Boston Marathon this afternoon were bombs, possibly placed in mailboxes or garbage cans, or hidden in some other fashion. Officials at the scene say two people are dead, including an 8-year-old, and some 100 are injured. Later, officials confirmed that a third explosion at the John F. Kennedy museum in Boston was probably linked to the attack. The Associated Press reports that two more explosive devices have been found on the marathon route and are being dismantled.

As rescue workers, bystanders, and runners rushed to try to help the wounded in the immediate aftermath, Boston closed its airport and shut down cellphone service. Several cities put their police forces on alert.

In New York, according to Deputy Police Commissioner Paul Browne, as many as 100 of the Critical Vehicle Response (CRV) police cars often seen on drills around the city, and always kept at the ready, were put in motion for immediate deployment to landmarks, hotels, and other possible targets of terrorist attack.

“These are some of the precautions we are taking until we can learn more,” said Browne.

In Washington, NBC reports, streets around the White House have been closed to traffic.

For the moment, no group or individual is being ruled out as the possible source of the attack, which could be the work of an individual, a group, or an organization. Although suspicion inevitably and immediately falls on radical jihadists, because of the record of the last many years, the police are painfully aware that there are no shortage of people capable of building bombs. Those terrorists with agendas that range from animal-rights to national-liberation movements, religious zealots, and criminals taking revenge.

What is certain is that Boston, New York, and many other cities will remain on alert for some time.