MARTINEZ — Contra Costa prosecutors have charged a Walnut Creek man with two counts of murder, alleging that he stabbed his wife and mother-in-law to death, then called 911 to report that he caught the killer in the act and fired his gun as the mysterious suspect ran off, authorities said.

Howard Wang, 43, was scheduled to be arraigned Tuesday afternoon. In a statement, Contra Costa County District Attorney spokesperson Ted Asregadoo said Wang has been charged with the Sept. 18 murders of his wife, Linlin Guo, 41, and his mother-in-law, Beimin Cheng.

The charges include special circumstances enhancements for multiple victims, Asregadoo said. If convicted on the charges, Wang faces a life sentence without the possibility of parole.Prosecutors also have charged Wang with making criminal threats and dissuading a witness, according to court records. Those charges stem from prior incidents of domestic violence at the home, where Wang allegedly either threatened his wife or prevented her from calling 911.

Officers responded to Kelondra Court about 11:45 p.m. on Sept. 18, after Wang called 911 to report finding his two family members dead or dying from stab wounds. According to authorities, Wang actually fired his gun off on the property, then claimed he’d shot at the escaping assailant. Police say they discovered the two women suffering from undisclosed injuries and pronounced them dead at the home.

Wang’s story fell apart almost immediately, authorities said. He was arrested Friday morning on suspicion of double murder, and prosecutors formally charged him on Monday.

Police did not say exactly how they came to determine Wang was a suspect. Police said only that they did so after the “initial investigation.”

Wang remained held in the Martinez Detention Facility on Tuesday without bail, according to jail records showed. Concord police and the Contra Costa County Sheriff’s Office helped with the investigation.