A hotel security officer who was on duty after Sean Combs brutally assaulted his longtime girlfriend, Casandra Ventura, took the witness stand on Tuesday at the music mogul’s federal trial.

It has been a year since 2016 footage of Mr. Combs striking and kicking Ms. Ventura in a Los Angeles hotel was broadcast on CNN. Now those images, which became a potent demonstration of the music mogul’s violence, are a centerpiece of his federal trial.

The hotel security officer, Eddy Garcia, is testifying under an immunity order after telling the government that he intended to assert his Fifth Amendment right to not incriminate himself.

Prosecutors are expected to delve into the events that followed the assault, which they have said involved Mr. Combs delivering $100,000 in a brown paper bag to purchase hotel security footage of the beating.

Mr. Combs is facing charges of sex trafficking and racketeering conspiracy, which involves accusations that the mogul engaged an inner circle of bodyguards and high-ranking employees to help him commit a series of crimes over two decades.

At least two of those criminal allegations — bribery and obstruction of justice — relate to the aftermath of the assault at an InterContinental hotel in Los Angeles.