Three drivers became ill Sunday, possibly due to ingesting the pot-laced brownies distributed by another MTS employee, according to MTS spokesman Rob Schupp. MTS officials requested emergency medical services and called in replacement drivers, he said.An internal investigation conducted by MTS determined all involved employees acted appropriately."I think they were responsible by pulling over once they felt dizzy and stopped the bus," said Celeste McClure, who rides the bus to work five days a week. When Oscar Rojas of San Ysidro heard about the incident, he had concerns for his safety and that of other riders. "Not a good thing," he said, shaking his head. "Makes me wonder what about the system, you know? Who's supervising these guys and what are we doing to correct this?" MTS Vice Chairman Ron Roberts told 10News he was informed of the incident Wednesday -- four days after it happened. "This could have been a major disaster," said Roberts. "It's not funny. It's not fun and it's irresponsible at the highest level." Roberts promised to get to the bottom of the situation and assured 10News that someone would be held accountable. "We're going to be very open with the public about what happened also," said Roberts. "The positive side to this incident is that our bus operators followed procedure to the letter and our supervisors acted quickly to protect our passengers and the public," said Paul Jablonski, MTS chief executive officer.All the involved employees were immediately placed on administrative leave while MTS investigated and administered drug tests, Schupp said. MTS also contacted police and the San Diego County District Attorney's Office, he said.Schupp said charges or disciplinary action against any employee had not yet been determined.The MTS confirmed "further investigation showed no additional bus operators had ingested any brownies."The employee that passed out the tainted brownies, baked by a roommate, was unaware they contained marijuana, 10News reported. . As part of a statement issued Thursday, officials said, "MTS has a comprehensive drug and alcohol policy, which tests all new employees and conducts random, unannounced tests throughout the year. The program exceeds guidelines set forth by the Department of Transportation."