A ransomware attack took ticket machines for San Francisco’s light rail transit system offline all day Saturday during one of the busiest shopping weekends of the year, but rather than shutting down, the agency decided instead to let users ride for free. By Sunday the system was once again running normally.

“We never considered paying the ransom. We have an IT team on staff who can fully restore all systems,” said San Francisco Municipal Transportation Agency spokesman Paul Rose.

SFMTA, known as Muni, reported that agents’ computer screens displayed the message “You Hacked, ALL Data Encrypted” beginning Friday night.

The San Francisco Examiner reported that riders of SFMTA’s Municipal Rail or “Muni” system were greeted with handmade “Out of Service” and “Metro Free” signs on station ticket machines. The computer terminals at all Muni locations carried the “hacked” message: “Contact for key ([email protected]),” the message read.

The hacker in control of that email account said he had compromised thousands of computers at the SFMTA, scrambling the files on those systems with strong encryption. The files encrypted by his ransomware, he said, could only be decrypted with a special digital key, and that key would cost 100 Bitcoins, or approximately USD $73,000.

That is almost certainly a conservative estimate of his overall earnings these past few months: My source said he was unable to hack another Yandex inbox used by this attacker between August and October 2016, “[email protected],” and that this email address is tied to many search results for tech help forum postings from people victimized by a strain of ransomware known as Mamba and HDD Cryptor.

Copies of messages shared with this author from those inboxes indicate that on Friday evening, Nov. 25, the attacker sent a message to SFMTA infrastructure manager Sean Cunningham with the following demand (the entirety of which has been trimmed for space reasons), signed with the pseudonym “Andy Saolis.”

“if You are Responsible in MUNI-RAILWAY !

All Your Computer’s/Server’s in MUNI-RAILWAY Domain Encrypted By AES 2048Bit!

We have 2000 Decryption Key !

Send 100BTC to My Bitcoin Wallet , then We Send you Decryption key For Your All Server’s HDD!!”

One hundred Bitcoins may seem like a lot, but it’s apparently not far from a usual payday for this attacker. On Nov. 20, hacked emails show that he successfully extorted 63 bitcoins (~$45,000) from a U.S.-based manufacturing firm.

According to a review of email messages from the Cryptom27 accounts shared by my source, the attacker routinely offered to help victims secure their systems from other hackers for a small number of extra Bitcoins.

In one case, a victim that had just forked over a 20 Bitcoin ransom seemed all too eager to pay more for tips on how to plug the security holes that got him hacked. In return, the hacker pasted a link to a Web server, and urged the victim to install a critical security patch for the company’s Java applications.