SAN FRANCISCO (CBS SF) — Police in San Francisco on Friday released chilling new details about how UPS shooting suspect Jimmy Lam used stolen weapons to target and kill three co-workers last week.

Authorities provided a new, more detailed timeline of the rampage, revealing how the disgruntled driver silently and purposefully targeted his colleagues.

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Police traced the movements of the gunman as he opened fire on three fellow UPS drivers the morning of June 14, before killing himself.

Police said the two weapons recovered at the scene were stolen, but that Lam only used one, a semi-automatic Mac 10 pistol that he fired 20 rounds from that day. They also found a backpack with a 30-round magazine inside.

Police also issued the first photo of Lam as well as pictures of the weapon he used.

“This shooting appeared purposeful and targeted,” said SFPD Commander Greg McEachern.

At 8:50 a.m., he stood shoulder to shoulder with his coworkers stretching and preparing for the day during their morning meeting at the Potrero Hill facility.

He opened fire without saying a word, mortally wounding Benson Louie and Wayne Chan.

He then shot two other employees who survived.

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After that, he calmly walked passed other UPS workers and left the building. At the corner of 17th and San Bruno, he shot Mike Lefiti several times.

Then Lam walked back into the UPS building and into the meeting room.

That was when San Francisco police officers arrived and saw him standing over his first two victims.

When Lam saw the officers, he put a gun to his head and killed himself.

Joseph Cilia of the UPS teamsters union says only four or five UPS employees in Jimmy Lam’s unit have been able to return to work since the shooting.

“Drivers are scared, frustrated, they’re on edge,” said Cilia.

Everyone else is out on worker’s compensation. For the drivers who are at work, there’s no escaping the story.

“A lot of customers are hugging them, greeting them,” said Cilia. “So it’s really hard when they’re constantly being reminded. They understand the customers are trying to be helpful, but it is a constant reminder.”

While authorities announced more details about the shooting, they still don’t know why Lam did it.

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“We have not established a motive for Mr. Lam,” said McEachern.