WATERTOWN (CBS) – The man charged with stabbing a tow truck driver who investigators say ran over and killed his mother in Watertown is being held without bail. Philip Horner, 38, could barely stand up as he pleaded not guilty Thursday afternoon to armed assault with intent to murder at his arraignment in Waltham District Court.

A psychiatrist gave brief testimony before Horner was sent to Bridgewater State Hospital for a mental health evaluation.

“After an evaluation by the court’s clinician today, Mr. Horner is being transported to a secure facility where he will be able to receive the medical attention that he needs,” said defense attorney Nicholas Louisa.

NEW VIDEO: Suspect in stabbing of tow truck driver in Watertown brought into court to face charges #WBZ pic.twitter.com/tXx6JQ0IpR — Jim Harrington (@jejharrington) September 20, 2018

Prosecutors say Horner repeatedly stabbed a tow truck driver Wednesday in Watertown Square, moments after Horner’s mother was hit by the truck. Benita Horner, 68, of Belmont, died at the scene.

According to court documents, Philip Horner was screaming to police “He ran over my mother” and of the multiple stab wounds, one went into the driver’s back and “plunged into him and then twisted with force.”

The driver, identified as 60-year-old Thomas Fogarty, was yelling, “I didn’t see anything, come on man,” according to court documents.

Witnesses described a chilling scene.

“He jumped in the truck, the cabin, and started stabbing and stabbing and stabbing him,” said witness Zeidan Taha, who called 911 from the scene. “And the driver, you know, opened the door and threw him to the floor. And then the driver went down, and then he kept chasing him, chasing him, and stabbing him, stabbing him.”

Fogarty, the tow truck driver, was stabbed five times. He was rushed to the hospital in critical condition. There was no update on his condition as of Thursday afternoon.

Neighbors of Benita Horner are stunned.

“Her son did not get the help that he needed and that’s the truth. I mean, I just can’t believe it,” said Anna Pizzuto.

Vyrna Napoli, another neighbor, said Philip Horner visited his mother frequently.

“I couldn’t go to sleep last night because I was so concerned about how he was going to be afterwards, because they were very close, I’m sure,” Napoli said of Philip Horner.

Benita Horner was trying to get her son help for his mental health, Pizzuto said.

“She said, ‘It take takes time. It takes time. I go here, I go here,'” she said.

Horner is due back in court on Oct. 10.