Deslauriers described that Suspect Two can be seen in videos putting a backpack down in front of the Forum Restaurant along the marathon route near Copley Square.

He also explained that the two men appeared to be walking along together up Boylston Street near where the explosions took place.

"We know the public will play a critical role in identifying and locating these individuals. Somebody out there knows these individuals," Deslauriers said.

The public were warned that the men are considered to be armed and extremely dangerous, and were asked that no one attempt to apprehend them personally. Anyone with any evidence should immediately come forward and contact law enforcement officials.

Suspect 1 was described as wearing a light-colored, collarless shirt underneath a dark-colored jacket and wearing a dark baseball cap.

It has also been reported by CNN that the FBI has other unreleased footage of the same two men showing that they acted "differently" to everyone else in the aftermath of the explosions. That footage reportedly shows the two suspects staying at the scene watching "the carnage unfold," a federal law enforcement official with knowledge of the investigation told CNN's Susan Candiotti.

"When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually," said the official, who has seen the video footage not publicly released.

"They acted differently than everyone else," he added.

The Boston Marathon twin bombings killed three and injured 178 others, according to reports, and is being considered the worst terrorist attack on U.S. soil since the 9/11 attacks. However, at this point the FBI has not determined whether the terror incident was domestic or foreign-based.

Earlier on Thursday, President Barack Obama delivered an address at an interfaith prayer service for the victims of the bombings at the Cathedral of the Holy Cross, focusing his speech on the redeeming power of grace in the face of tragedy.

Citing a poet who said Boston "is the perfect state of grace," Obama remarked: "And so we come together to pray and mourn and measure our loss. But we also come together today to reclaim that state of grace. To reaffirm that the spirit of this city is undaunted and the spirit of the country shall remain undimmed."

"The grace this tragedy exposed is the best of who we are," he added.

In addition to the theme of grace, the president also highlighted the theme of endurance and quoted Hebrews 12:1 (NIV) in his remarks. "Scripture tells us to run with endurance, the race that is set before us," he stated.

"To the injured, we will all be with you as you learn to stand, walk, and run again, yes, run again, for that I have no doubt you will run again," Obama said to a standing ovation.

"We will finish the race."

The president also quoted 2 Timothy 1:7, saying: "God has not given us a spirit of fear and timidity, but one of power, love and self-discipline, and that's the spirit you have displayed in recent days."

Anyone with information on the suspects should call 1-800-CALLFBI.

Here is a video showing the footage of the potential Boston bombing suspects: