SAN FRANCISCO (CBS SF) — Four suspects are in custody after a six-hour standoff Wednesday morning at a building in San Francisco’s Bayview District that houses a roughly 1,000-plant marijuana-growing operation, a police spokesman said.

The standoff began around 3:50 a.m. after suspects wearing ski masks robbed the facility, located in the 2100 block of Jennings Street, police Sgt. Michael Andraychak said.

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Three suspects fled on foot but were arrested about two blocks away, he said.

Hours later, officers remained at the scene and were “working under the belief that there are still people inside the building—possibly a suspect as well as employees,” Andraychak said.

He said, “We’re treating it as a possible barricaded situation, possibly a hostage situation.”

Around 8:30 a.m., tactical officers led four people from the building believed to be employees at the facility, Andraychak said.

They were unharmed, but Andraychak said he did not have details such as where they had been in the building or how they were freed.

The standoff continued until about 10 a.m. when the suspect surrendered after tactical officers entered the facility with a K-9 named “Tomak,” a 4-year-old Belgian Malinois.

“He did not want to be bitten by the dog,” Andraychak said.

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The suspect, whose name has not yet been released, was led away in handcuffs. Andraychak said it does not appear there are any more suspects at large.

“We believe we have all suspects in custody,” he said.

He said investigators have determined that at least one of the four suspects was armed with a gun at the time of the robbery.

The four employees who were removed from the building are being interviewed Wednesday afternoon, Andraychak said.

Police were initially calling the building a dispensary, but Andraychak clarified that it was actually a grow site with no storefront.

He said it is believed that the grow was permitted but that investigators are working to confirm that.

Andraychak said the building, which is about half a block long, contains about 1,000 plants in various stages of maturity, with some just inches high and others standing several feet tall.

This morning’s large-scale response included a SWAT team, the K-9 unit, crisis negotiators, firefighters, medical responders and parking and traffic officers.

During the standoff, several streets were shut down, and San Francisco Municipal Railway’s 54-Felton bus line was rerouted, Muni officials said.

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