It takes a long time and a lot of patience to run 26.2 miles. Now we are finding out that it is taking much longer to find the perpetrators of the Boston Marathon massacre. Patience remains a critical virtue.

On Thursday evening the FBI released pictures of two suspects in the crime. A video shows the two walking in a purposeful manner toward the sites where bombs were later exploded. One photograph shows one suspect placing his backpack in the spot that investigators think was the location of one of the bombs.

But the FBI does not know who these men are. Special Agent Rick DesLauriers, in a Boston press conference, asked the public’s assistance in identifying the two suspects.

Since Monday investigators have assembled myriad bits of evidence from the scene, including shrapnel, the twisted remains of a 1.6-quart pressure cooker, circuit boards that might be parts of a timer and fragments of black nylon surmised to be remnants of bags in which the two bombs were contained.

Key questions remain. What was the motive of the bombing? No person or group has claimed responsibility, leading some experts to downgrade the possibility that the attack was carried out by any known terrorist organization.

Why was the pressure-cooker bomb chosen? It has been used in attacks in South Asia and the Middle East associated with al-Qaida, and the method for making it has been published on a jihadist web site. But prior to Monday it had not been used in this country.

The association of the method of attack with jihadist literature could suggest that the perpetrators were motivated by al-Qaida, if not actually affiliated with it. But the lack of any claim of responsibility is more suggestive of volunteer domestic terrorists operating on their own. They are notoriously harder to identify and capture.

It required the examination of thousands of photographs and hours of surveillance recordings of the scene to come up with video images of the suspects.

Carrying the investigation forward will demand the same iron patience required to complete a marathon. Thursday’s break in the effort to identify the bombers provides confidence that the task will be accomplished in the end.