OAKLAND, Calif. -- An angry BART general manager has pledged a “rigorous investigation” of a second officer’s actions on the night Oscar Grant III was fatally slain by former transit police officer Johannes Mehserle.

BART General Manager Dorothy Dugger was responding to a videotape aired by KTVU that shows a second officer striking one of the men detained on the transit station platform early on January 1st.

“I take this new allegation of police use of unreasonable force extremely seriously,” Dugger said in a prepared statement released Saturday night. “I have directed (BART) Police Chief Gary Gee to conduct a rigorous internal affairs investigation into this officer’s actions.”

The video shows a BART police officer leaving one suspect on the ground and walking over to an area where a female officer is talking to three other men. The officer then appears to punch one of the men in the face.

"That was quite a shot," says Peter Keane a law professor with University of California Hastings College of Law. "That's a pretty brutal punch, knocks his head back, sends him to the ground."

After the punch, Grant can be seen putting his hands up. The two men flanking Grant sit down as the same officer appears to point a taser at them. The video then stops. Two officers arrive and that is when Mehserle allegedly shot Oscar Grant in the back, killing him. Mehserle now faces a murder charge.

Investigators say the officer who delivered the punch and who had his knee on Grant's neck during the shooting is not charged with anything, something Peter Keane disagrees with.

"If the district attorney is saying he's not going to charge any officer except Mehserle in my opinion, he's not doing his job," said U.C. Berkeley law school professor Franklin Zimring. Zimring also said the district attorney may be focusing on the shooting first, and "he may need that officer's testimony."

John Burris is representing Oscar Grant's family. He says he knew about the punch and believes it escalated the incident."His (the officer's) aggressiveness then caused the other officer to be aggressive, I believe," says Burris. "And that aggressiveness then ultimately led to the shooting, if you didn't have that level of conduct I don't believe the shooting would have taken place."

Alameda County District Attorney Tom Orloff wouldn't comment Friday on potential criminal charges about the BART police officer who threw the punch.