The city immediately notified the F.B.I. and took systems offline to keep the ransomware from spreading, but not before it took down voice mail, email, a parking fines database, and a system used to pay water bills, property taxes and vehicle citations.

At least 1,500 pending home sales have been delayed, too, according to a letter from a group of congressional lawmakers in Maryland requesting information on the attack from the directors of the F.B.I. and the Secret Service.

This week, the city put into place an offline fix to allow the transactions to proceed.

What was the threat?

A copy of a digital ransom note, obtained by The Baltimore Sun, stated that the city could unlock the seized files for a price: three Bitcoins (nearly $24,000) per system or 13 Bitcoins (about $102,000) for them all.

(The price of this decentralized, hard-to-track virtual currency fluctuates wildly. On the day of the attack, the ransom would have cost about $17,000 per system, or less than $75,000 for them all.)

“We won’t talk more, all we know is MONEY!” the note said.

Baltimore has released little else about the attack, citing a continuing F.B.I. investigation.