MARTINEZ -- The Contra Costa District Attorney's Office on Wednesday charged a 25-year-old Concord man with involuntary manslaughter in the death of a woman killed on the steps of the Martinez Chamber of Commerce as its offices were closed Sunday night.

Jason Castillo, who police say dated the 47-year-old victim Cecelia Kingsley off and on, is expected to be arraigned Thursday.

Police and prosecutors said the circumstances that led to Kingsley, a Martinez resident, dying of blunt force head trauma remain under investigation. A complete coroner's report is pending.

"It's still under investigation for multiple scenarios that could have occurred; an object hitting her, or her hitting her head on the concrete," Martinez police Sgt. Glenn Walkup said.

Involuntary manslaughter is the lowest of the homicide charges, and is appropriate when someone engages in criminal activity not amounting to a felony, but that endangers human life, deputy district attorney John Cope said.

Witnesses told police that Castillo hit Kingsley about 7:25 p.m. during a fight on the steps of chamber offices. Castillo was arrested a short time later on nearby Ferry Street.

Castillo and Kingsley were occasional transients. Though they both lived with their respective family members, they were known to spend times around the Amtrak station, located next to the chamber on Marina Vista Avenue, police said.

Kingsley's death marks the first homicide

Contact Malaika Fraley at 925-234-1684. Follow her at Twitter.com/malaikafraley.