California prosecutors filed a murder charge Friday alleging that a Sacramento man was lying in wait when he used one of two illegal assault rifles to kill a rookie police officer this week.

The charges filed by the Sacramento County district attorney include special circumstances that could subject 45-year-old Adel Sambrano Ramos to the death penalty. However, that decision is months away and Gov. Gavin Newsom has imposed a moratorium on executions.

Sacramento police said they were helping a woman collect her belongings from a Sacramento home as part of a domestic violence call when Ramos opened fire Wednesday evening, mortally wounding Officer Tara O’Sullivan, 26.

They said he continued shooting for hours, pinning down officers until they could use an armored vehicle to move O’Sullivan 45 minutes later.

He’s also charged with the attempted murder of a second officer, Daniel Chipp. The standoff with police lasted eight hours, with five officers firing their weapons.

Public defender Norm Dawson said he met briefly with Ramos but can’t comment until he receives more details in the case. Ramos is set for his first court appearance on Monday.

Prosecutors allege that Ramos had two AR-15 style rifles assembled from parts to create assault weapons that are illegal in California. One had a pistol grip and telescoping stock, the other a non-fixed magazine and with a pistol grip, according the charges.

Law enforcement officials have said such so-called ghost guns are a particular problem because they cannot be easily traced.

The special circumstances in the case accuse Ramos of knowing O’Sullivan was a police officer performing her duties, and killing her “while lying in wait,” which generally means that he took her by surprise before she knew an attack was coming.

“This is an ongoing investigation and we anticipate that additional charges may be added in the future,” prosecutors said.

Court records show that Ramos has a history of domestic violence restraining orders. He was wanted on a bench warrant issued nine days before the shooting for failing to appear on a charge that he battered a young woman last year.

“It would be inappropriate for me to comment any further until I have a chance to review the materials,” said Dawson, a supervising public defender. “It will be several months before we entertain entering any plea.”

Ramos also has not responded to an interview request from The Associated Press

O’Sullivan is the first Sacramento police officer to be killed on the job in two decades.