The YouTube shooter purchased her gun three months ago — and blasted through at least one magazine when she opened fire at the company’s headquarters this week, police revealed Thursday.

Nasim Aghdam legally purchased a 9mm Smith & Wesson handgun from an unnamed gun store in the California city on Jan. 16, San Bruno Police Commander Geoff Caldwell told the Mercury News.

It’s unclear exactly how many bullets Aghdam, 39, fired when she stormed the video-sharing giant’s San Bruno campus Tuesday — wounding three people before killing herself — but one spent magazine was found at the scene, he said.

“Halfway through she exchanged magazines and continued shooting until she turned the gun on herself,” Caldwell said.

Aghdam visited a nearby gun range before the shooting — and staff at the facility reached out to cops to say they recognized her after seeing news reports of the incident on TV, he added.

“She went there and shot,” Caldwell said, according to the Mercury News.