Boston Marathon Bombings: Surveillance video spies man on cellphone, with black bag, placing it at site of 2nd blast

Lu Lingzi, a 23-year-old student from Shenyang, China, and a graduate student at Boston University, was identified as the third victim killed in the bombing attack at the Boston Marathon on April 15, 2013.
People react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013.
AP Photo/The Daily Free Press, Kenshin Okubo

(CBS/AP)  - CBS News correspondent Bob Orr reports sources are "very encouraged" by developments in the Boston Marathon bombing investigation, and says authorities are focused on a man seen in surveillance video at the site of the second explosion, with a black backpack that he placed on the ground while he was on a cell phone call.

PICTURES: Boston bombing victims

PICTURES: Explosions near Boston Marathon finish line

The FBI has scheduled at 5p ET news conference.

Orr reports the man was wearing black jacket, grey hoodie, and a white baseball cap on backwards. He says it is not certain if the man in question is a witness or a suspect.

He also reported that while the man was on a cell phone, the first explosion occurred closer to the race finish line - and at that point, the individual mingled back into the crowd.

Authorities have been collecting photos and videos from surveillance cameras and other digital media since the twin bombings Monday that killed three people and wounded more than 170 others.

Complete coverage of Boston Marathon bombings on Crimesider