It was the eighth inmate death reported at the jail since 2010.
The woman, 51, was found unconscious about 10:30 p.m. Friday, said Shari Ireton, spokeswoman for the Snohomish County Sheriff's Office.
She had been housed in the jail's medical unit since her arrest two days before, Ireton said. Corrections staff were checking on her about once an hour.
The staff who found the woman unresponsive started resuscitation efforts and summoned medics. She died at the scene.
The sheriff's office sent out a news release late Monday afternoon. Officials aren't releasing additional details about the woman until her family is notified of her death, Ireton said.
The sheriff's major crimes detectives are investigating, which is a standard practice, she said. The Snohomish County Medical Examiner's Office will determine the woman's cause of death.
The sheriff's office also plans an internal review to determine if "there were any policy or procedural violations," Ireton said.
At least seven other inmates have died at the jail in recent years. At least two of those deaths involve pending legal claims alleging that inmates were denied basic medical care. Another internal investigation just wrapped up, and prosecutors still are reviewing a separate case from last year.
Experts with the U.S. Department of Justice are scheduled to visit and review conditions and policies at the jail in the coming weeks. The assessment was requested when County Executive John Lovick was still sheriff, earlier this year.
County officials are particularly interested in whether medical care at the jail measures up.
Lyndsey Elizabeth Lason, 27, suffocated at the jail in 2011 when her infected lungs slowly filled with fluid. Other inmates said Lason had pleaded for medical care. A $10 million wrongful death claim is pending.
Michael Saffioti, 22, died at the jail in July 2012 from bronchial asthma triggered by severe allergies. His family has hired a Seattle attorney to press for answers. He was in jail for marijuana possession.
The last inmate death was a methamphetamine overdose in February. The other deaths included factors such as heart problems and suicide.
The jail houses about 1,200 inmates on an average day. The sheriff's office took over operations in 2008.
Rikki King: 425-339-3449; email@example.com
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