A top law enforcement official said the pressure cookers in the two bombs in Chelsea on Saturday night were filled with “fragmentation materials.” The bomb that exploded, at 23rd Street, was filled with small bearings or metal BBs. A second device on 27th Street that did not explode appeared to be filled with the same material, the official said…

An expert on improvised explosive devices used by terrorists around the world said that a device constructed with a cellular phone as a timer and Christmas lights as an initiator would indicate a higher-than-average competence than what is usually found in the United States. “Most of what we see in the United States is a pipe bomb with black powder or smokeless powder or a simple hobby fuse,” said the expert, who spoke on the condition of anonymity because he does sensitive work for government agencies. “This would be the high end of sophistication for I.E.D.s in the United States.”

The recovery of the second device provides investigators with a host of potential leads, from the phone and other materials inside the homemade bomb to the pressure cooker itself — including possibly determining when and where it was purchased — to any possible fingerprints on the device, its components or the tape that held it together.