SAN FRANCISCO -- San Francisco's Municipal Transit Agency is launching an internal investigation after video of a fight between two bus passengers that the vehicle’s driver never reported popped up on YouTube.

But as KTVU learned after further investigation Thursday night, the altercation between the two passengers might not such an unusual occurrence on the transit system.

Muni says it learned of the incident yesterday from the amateur video posted on YouTube. It shows two passengers getting into a fight, though the exact cause of the fight remains unclear.

It appears that the altercation took place Wednesday morning at around 10 a.m. on the 20 Columbus line thru Chinatown.

Muni passenger Kristine Doss said the incident wasn’t unusual for muni.

“I wasn't surprised at all. It's typical, people fighting for spots on the bus,” said Doss. “I've seen people punching, throwing things, pulling hair; people throwing groceries at the driver because he won't let them on.”

She had seen the YouTube video and pointed out it was passengers who tried to break up the fight.

“I'm concerned that the passengers had to do something,” explained Doss “It shouldn't be the passengers [who] police the buses.”

Some passengers would like to see Muni provide better security on its buses such as undercover officers or random patrols.

“It'd be nice if the drivers had some training in breaking up the fights and putting a stop to those sorts of things,” said passenger Sarah Coleman.

Muni spokesman Judson True said drivers are not supposed to intervene in a fight, but they are required to follow a certain protocol when there's a physical altercation.

“In general, our operators are required by our rule book to report any assault on a vehicle or anything that's unduly disruptive,” said True. “Then our central control will contact the police department, which would respond.”

Muni said it appeared that the driver in this incident did not report the altercation. Transit officials said the incident took place in the back of what's called an articulated bus. Those vehicles are 60 feet long, so it's possible the driver was unaware that a fight was taking place on board.

If we do determine for example in this incident that the operator didn't respond correctly, we'll handle that appropriately,” explained True.

Muni said if disciplinary action against the driver is warranted, the action can range from a warning to suspension.

Muni said its most recent records indicate that there were 240 assaults reported between April of 2008 and June of this year. But the agency had no idea how many incidents go unreported.

“Sometimes the driver will pull over and tell the people to get off or he'll pull over and say we're stopping till the police come,” said Doss. “I've also seen the driver keep on going.”

Muni said investigators interviewed the bus driver Thursday, but would not reveal what was said because the conversation was part of their ongoing investigation that should be completed in a few weeks.