San Francisco commuters travelled for free on Saturday after a hack attack disabled ticketing machines on the public transport network.
The city's transport authority shut down the ticketing system at stations, where screens had displayed a message that said: "You Hacked, ALL Data Encrypted. Contact For Key(email@example.com)ID:681 ,Enter."
The BBC said Yandex is a Russian internet company that offers email and social networking tools. It also reported that the hackers asked for a ransom of 100 Bitcoin, which is equal to about US$70,000.
Trains managed by the Municipal Transportation Agency, known as Muni, were not affected. The machines were back to normal by Sunday.
"There has been no impact to the transit service, to our safety systems or to our customer's personal information," a spokesman told the BBC.
"The incident remains under investigation, so it wouldn't be appropriate to provide any additional details at this point."