A Millbrae man was arrested Thursday night after an investigation into online threats and gunfire inside an apartment building led to the discovery of an illegal assault rifle and suspected fentanyl, authorities said.

San Mateo County sheriff’s deputies identified the suspect as Anthony D. Roberts, 27.

On Wednesday, deputies began investigating after receiving reports that Roberts had allegedly sent messages suggesting potential violence. Roberts reportedly included a photo of a bullet hole in his apartment wall, the Sheriff’s Office said.

Deputies obtained an arrest warrant, search warrant, and a gun violence restraining order, a press release noted.

Roberts was taken into custody outside his apartment in the 100 block of N. Rollins Road without incident, officials said.

During the s

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