Police: Stolen backpack sparked guard’s attack at Coliseum

A security guard apparently attacked a woman at an Oakland Athletics game Monday. A security guard apparently attacked a woman at an Oakland Athletics game Monday. Photo: Screenshot From Video By Shane Siegrist Photo: Screenshot From Video By Shane Siegrist Image 1 of / 4 Caption Close Police: Stolen backpack sparked guard’s attack at Coliseum 1 / 4 Back to Gallery

The intensity on the field between the Giants and A’s was tame Monday night compared with the fisticuffs that took place in the upper deck of the Coliseum.

A video of the violence appears to show a security guard assaulting a fan, prompting the A’s to open an investigation.

The guard involved in the fight was employed by a sub-contractor, USSA, which was hired by Staff Pro, the company that handles security for A’s games, team spokeswoman Catherine Aker said.

USSA was taken off security duty for Tuesday night’s Bay Bridge series game as the A’s mulled whether to use it at future games.

“We’re evaluating that right now — I can’t confirm anything beyond tonight’s game,” Aker said Tuesday.

In video of the incident, a female security guard appears to be trying to beat a woman in the third deck, but other security guards intervene and pull away their colleague.

Witnesses posted to social media that the attack appeared unprovoked and that the security guard was booed when she returned to the area.

In a statement Monday night, the A’s said they were investigating the incident.

“Violence in any form is not acceptable. We take this incident seriously,” the organization wrote on Twitter.

Aker later released a statement, saying, “This individual will not return to work A’s games. Fan safety is our utmost concern.”

The incident started when the security guard’s backpack was taken about 9 p.m., according to Officer Johnna Watson, a spokeswoman for the Oakland Police Department.

“The backpack was located by another security guard in possession of a fan in another section. When that security officer confronted the fan, a fight ensued,” Watson said in a statement.

Security members then escorted the fan from the stadium, according to Watson. The backpack was recovered, but some items were missing, she said. She added that the incident was under investigation and did not specify whether any arrests were made.

The security guard has been fired, according to a Staff Pro spokeswoman, who added that the woman worked for the company for a little less than a year and passed a drug test and background check before she was hired.

“We regret that the actions of a rogue employee affected Oakland fans in a negative way,” Sandra Gravanti, vice president of risk management for Staff Pro, said in a a statement.

Monday night’s game, attended by 38,391 fans, was won by Oakland 8-5.

Chronicle staff writer Michael Bodley contributed to this report.

Filipa Ioannou is a San Francisco Chronicle staff writer. Email: fioannou@sfchronicle.com Twitter: @obioannoukenobi