(KTLA) – Two people were found fatally stabbed in Southern California Monday afternoon, and officers say the attack was witnessed by a colleague on a Zoom call.

The incident was reported Monday afternoon on North Marengo Avenue in Altadena. Officers arrived to find a man and his sister dead from apparent stab wounds.

“When they arrived, they found an elderly male deceased in the driveway,” said Lt. Barry Hall of the Sheriff’s Department. “They checked inside of house and found an elderly female dead.”

Authorities say the woman was on a Zoom call with a colleague when the attack started. 

“The person that the lady was still talking to on the Zoom with when the male was dragged out of the house, that alerted her and the suspect went inside and attacked her,” Hall said.

The virtual witness, who saw the attack on both the man and the woman, called 911, LA County deputies said in a press release.

As detectives were investigating, they learned a vehicle was missing from the residence, the department said.

“Since we arrived here at the location, a person of interest returned to the scene in the victim’s vehicle and he has since been detained,” Hall said.

That person of interest, 32-year-old Robert Cotton, was later booked on suspicion of two counts of murder, the department said.

Cotton told authorities he lived at that residence. Neighbors told Nexstar’s KTLA at the scene that the alleged suspect is related to the two victims.

The Washington Post reports that the vicitms were siblings.