A 15-year-old is accused of stabbing his mother and fatally stabbing his grandmother at their home in Selden, according to Suffolk County Police.

Police said the suspect's mother, Vanessa Chendemi, 36, was stabbed multiple times but still managed to run from the home at 802 Old Town Road around 9:45 p.m. Outside, she flagged an innocent bystander who picked her up in her car and drove her to safety.

Both women called 911.

When officers arrived at the home, they found the suspect's grandmother, Concetta Chendemi, 56, inside the residence with stab wounds. She was pronounced dead at the scene by a physician assistant.

The mother was taken to a hospital in stable condition and is expected to survive.

"I can't even imagine what that family is going through right now," neighbor DJ Cortez said. "It's terrible."

The 15-year-old fled the scene after the stabbing, police say.

K-9 officers canvassing the area arrested the boy at Dare Road and Buckskin Lane roughly an hour after the initial call, about one-third of a mile from the home.

Police recovered what they said were two chefs' knives used in the attacks.

"The knives were recovered at the scene just west of us," Suffolk County Police Department Homicide Squad Det. Lt. Kevin Beyrer said.

Police are now investigating what could have sparked the vicious attack.

They initially believed it had to do with something on the teen's computer.

"Looks like it was an argument over some content on his computer," Beyrer said. When asked to elaborate on what that could have been, Beyrer told News 12: "We're not going to elaborate on that."

Police charged the boy with murder and attempted murder. Officials are not providing his name because of his age.

He is expected to be arraigned later today.