An 18-wheel truck heading toward Somerville yesterday killed a woman on Somerville Ave. in a hit-and-run accident in the vicinity of the Porter Square Shopping Center, Cambridge police said.

Police responded to a report at 5:15 p.m. yesterday to find the woman severely injured. She was brought to Cambridge Hospital where she was pronounced dead, according to Cambridge Police spokesperson Frank T. Pasquarello.

The woman, who police did not identify pending notification of relatives, was a Cambridge resident in her 50's, Pasquarello said in an interview. She was not affiliated with Harvard University.

The truck was described by witnesses as an 18-wheel, yellow and white truck with lettering on the back. It continued down Somerville Ave. toward Somerville. As of last night, the vehicle had not been tracked down.

"We don't even know if the vehicle operator knew he hit the woman," said Pasquarello. It is unclear how fast the truck was traveling when it hit the woman.

Witnesses were present but "thev're not able to provide much more than what we've given out," said Pasquarello.

The incident was the first hit-and-run fatality this year and the first in recent memory, Pasquarello said. "I can't recall the last one we've had," said the 18-year veteran of the department.

Cambridge police urged anyone with information about the incident to call (617) 349-3301.