UNION CITY (CBS SF) — While investigating a robbery and possible shooting, police stumbled upon a massive illegal marijuana grow in a Union City warehouse Monday night, according to police officials.

Capt. Travis Souza said around 1:20 a.m. Monday, officers responded to a security guard’s report of an attempted robbery at a warehouse on Zwissig Way. The guard said he saw around eight suspects wearing dark clothes and leaving the warehouse with large bags full of what he suspected was marijuana, according to Souza.

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The guard that he thought the suspects might have shot at him, as he heard loud “pops” as he drove away.

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“We believe the victim was shot at and the vehicle was hit, causing a bullet hole in the rear bumper and a flat tire. The victim was not hurt otherwise, though,” Souza told the Mercury News. Souza also noted that the guard saw the suspects leave the area in two separate vehicles.

While searching for clues, police looked inside the 15,000 sq. ft and discovered the pot grow. Souza did not know how many plants there were but noted that the operation was massive,

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As of Tuesday, police were still investigating the robbery and grow operation. Investigators had not yet determined who owned the grow house.