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Sources told 10News an MTS employee brought brownies to the MTS bus terminal next to Petco Park at 12th and Imperial avenues last Sunday. The brownies were baked by the employee's roommate, and sources said the employee was unaware they were laced with pot.

Later in the day, four bus drivers complained of dizziness while on their respective routes and had to pull over, according to MTS officials. Replacement drivers were brought in, MTS officials told 10News.

Sources told 10News two of the drivers -- both female -- were hospitalized.

Four bus drivers and an employee were placed on paid leave, MTS officials confirmed.

10News obtained an inside memo that was sent out to all board members Wednesday evening from MTS CEO Paul Jablonski.

The memo read in part: "The source of the illness has been traced back to baked goods distributed by one fellow employee… all employees involved in this incident are on leave pending drug testing."

MTS official Rob Schupp said an internal investigation would determine if authorities are notified or will be involved in the investigation process.