San Francisco made its public transit free for a day after it was hacked

San Francisco residents were able to take the bus and trolley for free on Saturday after the city's computer systems for taking fares was apparently hacked.

SF Muni computers displayed the message "You Hacked, ALL Data Encrypted," the San Francisco Examiner reported on Saturday. 

Machines where riders could fill up their fare cards had signs attached that said "out of service" and "free Muni" on Saturday.

The attack seems to be an example of ransomware, where a computer system is taken over and the users are locked out until a certain amount of money is sent to the attacker. The Muni hack reportedly included an email address where Muni officials could ask for the key to unlock its systems.

Outages started happening on Friday afternoon but the bulk of the delays started on Saturday, according to a local ABC News affiliate. The extent and the identity of the perpetrator of the hack isn't known yet.

"We are currently working to resolve the situation," a Muni spokesperson told the Examiner. "There is an ongoing investigation and it wouldn't be appropriate to provide additional details."