At least 13,000 Chicago motorists have been cited with undeserved tickets thanks to malfunctioning red-light cameras, according to a 10-month investigation published Friday by the Chicago Tribune. The report found that the $100 fines were a result of "faulty equipment, human tinkering or both."

According to the investigation:

Cameras that for years generated just a few tickets daily suddenly caught dozens of drivers a day. One camera near the United Center rocketed from generating one ticket per day to 56 per day for a two-week period last summer before mysteriously dropping back to normal. Tickets for so-called rolling right turns on red shot up during some of the most dramatic spikes, suggesting an unannounced change in enforcement. One North Side camera generated only a dozen tickets for rolling rights out of 100 total tickets in the entire second half of 2011. Then, over a 12-day spike, it spewed 563 tickets—560 of them for rolling rights. Many of the spikes were marked by periods immediately before or after when no tickets were issued—downtimes suggesting human intervention that should have been documented. City officials said they cannot explain the absence of such records.

City officials and Redflex Traffic Systems of Arizona, the report said, "acknowledged oversight failures and said the explosions of tickets should have been detected and resolved as they occurred. But they said that doesn't mean the drivers weren't breaking the law, and they defended the red light camera program overall as a safety success story. The program has generated nearly $500 million in revenue since it began in 2003."

The city said it didn't know of the spikes, which began in 2007, until the Chicago Tribune provided it with evidence, the report said.