Law enforcement and intelligence authorities investigating the Boston terror bombings suspect al Qaeda terrorists were behind the blasts that killed three and injured scores although so far there is no firm evidence.

A U.S. official familiar with briefings on the probe into the blast said investigators remain stymied in determining who was behind the bombings near the finish line of the Boston Marathon Monday.

So far there has been no claim of responsibility, something that in the past was a characteristic of al Qaeda’s attack methodology. Senior jihadists monitored by Western intelligence agencies on the Internet remained silent in the hours after the bombings, officials said.

Investigators also are checking recent departures from Boston’s Logan airport for clues.

Richard DesLauriers, FBI special agent in charge in Boston, said in an an afternoon press conference that among the items recovered form the bombing were fragments and BB’s “possibly contained in a pressure cooker device.”

The devices were placed in a dark-colored nylon bag or backpack.