UC Berkeley is looking into allegations that a staff member was involved in an altercation during protests last week over a scheduled appearance by right-wing provocateur Milo Yiannopoulos, campus officials said Monday.

The accusations against the staffer, who works in the University Development and Alumni Relations Department, emerged on ultraconservative fringe websites and on social media following Wednesday’s destructive protests.

While details of what — if anything — happened could not be confirmed, the worker quickly became the target of an online campaign, with some people calling for his arrest and others harassing and threatening him and his family.

“We are aware of the allegations that focus on a current UC Berkeley staff member, and the campus police force is working in close concert with the FBI in an ongoing investigation in this matter,” said UC Berkeley spokesman Dan Mogulof.

The Chronicle is not naming the worker because he has not been arrested and has not been named as a suspect in a crime. He declined to comment Monday, saying he was consulting an attorney.

The way the episode has played out online sheds light on a virtual world in which disorganized bands of social media users become energized and converge on a person or cause.

Yiannopoulos, a Breitbart News editor who revels in provoking outrage with often racist and misogynistic rhetoric, has inspired such activity in the past. In July, he was booted off Twitter after igniting a storm of racist attacks from Internet trolls against African American comedian Leslie Jones.

The alleged incident at Berkeley happened as hundreds of demonstrators assembled on the campus Wednesday night to protest Yiannopoulos.

What started as a largely peaceful gathering outside the student union building on Sproul Plaza turned destructive when scores of black-clad anarchists joined the event. Some smashed windows and set fires, forcing UC Berkeley to cancel the speech.

It was in the aftermath of this chaos that online sleuths opposed to the protests accused the UC Berkeley staffer of being in a violent altercation with an unidentified supporter of President Trump.

The staffer was quoted by name in at least one media account of the campus protest, but it is not clear whether that’s how he came to the attention of his online accusers.

The evidence of the alleged altercation — later circulated on social media — amounts to a screenshot of a Twitter account that purportedly belongs to the staffer and is now protected from public view. The Chronicle could not verify if the account belongs to the UC Berkeley worker.

One tweet shows a man lying on his back and reads, “so, this guy. He was in a group of 4-5 MAGA chuds who were standing against the barricades berating protesters as we marched past. 1/.”

The tweet — using the acronym for Trump’s slogan, Make America Great Again — appears to be the first in a series, but a screenshot circulating online only shows a partial fifth tweet followed by two more. Any context and details of what was described in the other tweets remains in question.

“Some good samaritans come and pull me off of him and, as always happens in thee (sic) cases, *i* take a couple of weak shots to the face. 6/,” another tweet reads.

The tweets inspired a wave of anger — and threats — on social media from people upset over what happened in Berkeley, with many making unsubstantiated assumptions about the staffer’s role in the protest.

On Twitter, users urged the staffer’s arrest and “doxxed” him by gathering and publishing private information about him, including where he lives and where he can be found in the future. Some spoke of a bounty.

Wednesday’s destruction grabbed headlines around the world, prompting debate over the handling of the event by the university and outrage over the actions of the anarchists.

The destruction led a range of conservatives — including Yiannopoulos and President Trump — to denounce UC Berkeley and its students. A day after the protests, Trump used Twitter to threaten to pull federal funding from the university, even though the university had allowed Yiannopoulos to speak before the violence occurred.

Yiannopoulos has vowed to return to Berkeley to speak in the “next few months.”

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky