The four victims — two juveniles and two adults — were taken by ambulance to a hospital, where the teen, who was shot in the chest, died, authorities said.
The sheriff's office provides policing services under a contract with Windsor. Sheriff Steve Freitas did not respond to several messages Sunday seeking comment.
Windsor police said a suspect in the shooting fled in a blue Nissan compact. They described him as a black man wearing a black shirt and sweatpants.
The vehicle, driven by a female and possibly holding two other passengers, was last seen eastbound on Windsor River Road.
Focha on Sunday said she could not provide additional details about the suspect or whether additional people were being sought.
Focha also declined to elaborate on her statement that it was "unknown if the violence was gang-related."
Investigators worked Sunday morning behind yellow police tape which cordoned off the 500 block of Windsor Ave, just north of Windsor River Road.
Two pickups, also behind yellow tape, were parked in the driveway of the gray-green split-level house at the corner of Windsor River Road and Windsor Avenue.
"That's a damn shame," said Al Stoller, the next-door neighbor, when told of the fatality.
Stoller, who said he vaguely knows the residents, said he heard music from an apparent party starting up Saturday afternoon, describing it as "nothing obnoxious."
Stoller said he heard nothing from his bedroom early Sunday, noting that his grandson was playing a loud video game.
Dean Birrell, who lives across Windsor River Road from the house, said he woke up at 1 a.m. but did not hear any shots. At least 10 police cars and fire engines responded to the scene, he said.
Birrell, who has lived in the working class neighborhood for almost 30 years, said it has been pretty quiet for the past 15 years or more.
But before Windsor incorporated in 1992, Birrell said the neighborhood was "gang central" with three or four gang houses near his home.
Windsor, now a suburban town of 27,000 residents known for its tidy Town Green, was known as "Poor Man's Flat" prior to its incorporation.
Birrell said another resident told him he heard about six loud shots.
"It's a little freaky," said neighbor Leslie Jones, who moved to Windsor three months ago.
Jones said she heard nothing overnight and woke up at 6 a.m.
The case is being investigated by the Sonoma County Sheriff's Office's Violent Crimes Unit. Anyone with information about the shooting is asked to contact Detective Gary Freitas at 565-2185.
Staff Writer Derek Moore contributed to this report. You can reach Staff Writer Guy Kovner at 521-5457 or firstname.lastname@example.org.