'Cold-Blooded Mowing' of Victims Seen in Surveillance Video From Pulse Nightclub

— -- Federal authorities have recovered video from security cameras inside Orlando's Pulse nightclub that shows shooter Omar Mateen firing his assault rifle and systematically killing innocent victims, sources told ABC News.

The violence depicted in the video can only be described as "cold-blooded mowing, walking through the rooms mowing people down," one source said.

Sources characterized the video -- pulled from cameras deployed in multiple rooms of the nightclub -- as chilling and unnerving. The video portrays Mateen as a relentless assassin, reloading his weapon more than once as he murdered his victims.

The video could prove critical in helping law enforcement understand precisely how the killing took place.

Mateen killed 49 people and injured 53 others after opening fire inside Pulse early Sunday morning.