April 18, 2013  -- The FBI today released images of two suspects in the Boston Marathon bombing the bureau still considers to be "armed and extremely dangerous."

At a press conference, FBI special agent in charge Richard DesLauriers referred to the two men as Suspect 1 and Suspect 2 and said Suspect 2 was spotted dropping a bag before a bomb exploded at the second bomb site. The two men appear to be "associated," DesLauriers said.

DesLauriers asked the public for their help identifying the suspects, but asked the public not to approach them. Though the men are considered armed and dangerous, the FBI said they still do not believe there is an imminent danger in the area.

A few hours later, an FBI official cited a large volume of calls as a result of the photos and DesLauriers' appeal for help.

A source close to the investigation told ABC News authorities have "good confidence" that the men in the photos perpetrated the crime. The degree of confidence, the source said, comes from a "revealing" reaction Suspect 2 had to the detonation of the first bomb, which was captured on surveillance video.

Before that, the two are seen in surveillance video walking together before Suspect 2 stops at what would become the second blast site while Suspect 1 continues in the direction of the first blast site, the source said. A senior law enforcement official said one of the suspects is seen making a phone call around the time of the explosions, leading investigators to believe it may have triggered the blast. Investigators are trying to trace that call back to the phone used by the suspect.

"Somebody out there knows these individuals as friends, neighbors, co-workers, or family members..." DesLauriers said. "Though it might be difficult, the nation is counting on those with information to come forward and provide it to us."

"Identifying those responsible is now our highest priority," he said.

Three people were killed and more than 170 wounded in dual blasts near the finish line of the Boston Marathon Monday. No group, foreign or domestic, has claimed responsibility.

PHOTOS: Terror at the Boston Marathon

Pictures Latest Major Lead After Bomb Parts, Phone Calls

The FBI released the pictures of the suspects after three days of collecting a mountain of evidence, from spectator photos and videos to cell phone call logs and physical evidence of a partially exploded bomb at one of the blast sites.

That bomb, which authorities say was housed in a common pressure cooker, also appears to have contained shrapnel, wires, a battery and a small circuit board.

In a new development, today store owners in Massachusetts and across the border in New Hampshire told ABC News federal agents questioned them about sales of small batteries that are specifically used in DuraTrax brand radio-controlled cars – parts of which may have been used to build the bombs.

READ MORE: Boston Marathon Bomb Made With Toy Car Parts?

While they follow the additional leads, by releasing the photos the FBI hopes to drop an "electronic dragnet" over the suspects, according to a senior law enforcement official.

"In an instant, images will be delivered into the hands of millions around the world," DesLauriers said in the press conference. "We know the public will play a critical role in identifying and locating these individuals."

The FBI asked that anyone with information on the two men to call 1-800-CALL-FBI or visit a special FBI website at bostonmarathontips.fbi.gov.

ABC News' Jack Date, Aaron Katersky, Angela M. Hill, Ned Berkowitz, Randy Kreider and Rym Momtaz contributed to this report.

COMPLETE COVERAGE: Terror at the Boston Marathon

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