WATERTOWN, MASS. (WHDH) - The man accused of attacking and repeatedly stabbing a tow truck driver who fatally struck his mother in Watertown Wednesday morning has been ordered held without bail pending a psychiatric evaluation.

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Philip Horner, 38, pleaded not guilty in Waltham District Court Thursday to a charge of armed assault with intent to murder in connection with the stabbing of a tow truck driver who hit and killed Benita Horner, 68, of Belmont, as she was crossing the street, according to a spokeswoman for Middlesex District Attorney Marian T. Ryan’s Office.

The victim of the stabbing, 60-year-old Thomas Fogerty, remains hospitalized in Boston and is said to be in critical condition.

Officers responding to the intersection of Galen and Watertown streets about 11:20 a.m. found Benita Horner in the road and Fogerty suffering from multiple stab wounds.

Horner retaliated for her death by attacking Fogerty and stabbing him at least five times, Ryan said.

Zeidan Taha, who captured video of the graphic knife attack, says Horner jumped into Fogerty’s cab, fell to the ground, pulled a knife from a fanny pack and then stabbed him in the side and back.

Horner could be heard screaming, “no, no, no,” as his mother’s body laid lodged under the wheels of the tow truck.

The video shows Fogerty stumble away before collapsing on a sidewalk as frightened witnesses gasped loudly.

State and local officers moved in quickly, subdued Horner and took him into custody.

Horner’s attorney Nick Louisa argued his client was not fit to be arraigned, citing ongoing mental health issues.

Horner must complete 20-day psychiatric evaluation at Bridgewater State Hospital before he returns to court in October.

An investigation is ongoing.

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