The San Francisco Municipal Transportation Agency gave customers free rides on Friday and Saturday after the agency's computer system was breached by a group of hackers.

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According to the San Francisco Examiner, the MUNI system was attacked on Friday afternoon. "Out of Service" messages were written all over the screens of the ticket machines in pink letters which were superimposed with white printed sheets that read "Metro Free" signs.

Computer screens at MUNI stations displayed a message saying: "You Hacked, ALL Data Encrypted. Contact For Key(cryptom27@yandex.com)ID:681 ,Enter," The Verge reported.

MUNI Spokesman Paul Rose noted that his agency was "working to resolve the situation" as soon as possible but declined to provide any further details.

The hackers' motive is unclear. The hackers have confirmed via email that they are seeking a deal with MUNI to undo the damage.

"We don't attention to interview and propagate news ! our software working completely automatically and we don't have targeted attack to anywhere ! SFMTA network was Very Open and 2000 Server/PC infected by software ! so we are waiting for contact any responsible person in SFMTA but i think they don't want deal ! so we close this email tomorrow!" the hackers said.

In a similar case in the past, Morphus Labs linked a hacker to a ransomware strain called Mamba in September, which employs tactics similar to those demonstrated against MUNI.

This is not the first California organization to face such an issue. Earlier this year, the files of Hollywood Presbyterian Medical Center were held for a ransom of around $3.6 million. These type of ransomware attacks mostly occur on relatively unsafe systems when a malicious file is downloaded onto a computer and executed. Once the victim pays the demanded ransom, the files are decrypted.