Ransomware, a type of malware, encrypted the data on the Democrats' server and demanded an online payment in exchange for a key code to unlock the system, according to a statement issued Friday by Senate Democratic Leader Jay Costa.
"Officials from the caucus have been in contact with law enforcement to investigate the incident and are working with Microsoft to restore the IT system. There is currently no indication that the caucus system was targeted or that any data has been compromised," Costa said in the statement.
The network provides computer services like email access, web hosting and file storage for all 16 Democratic senators and their employees, according to Stacey Witalec, Costa's press secretary. As of Saturday evening, the caucus' website remained down, as did the individual websites of all 16 state senators.
Websites for Democratic members of the Pennsylvania House of Representatives remained accessible Saturday.
The FBI's Philadelphia field office confirmed Saturday that it is involved in an investigation of the cyberattack, which appeared limited to the Democratic Caucus.
"At this time, we're only aware that the PA Senate Democrats are affected," FBI spokeswoman Carrie Adamowski said in an email.
Witalec declined to comment on how much the attackers were demanding for the key code to unlock the server, citing the FBI's ongoing investigation. She said that Democratic staffers were working "around the clock" with Microsoft to assess the malware.
When asked whether the Democrats planned to pay the undisclosed sum, she said, "At this point we are working with Microsoft to see where we're at."
Democratic Governor Tom Wolf's computer systems remained unaffected, according to Lt. Governor Mike Stack's office, who said the gubernatorial network was entirely separate from that of the caucus.
All Senate Democratic offices remained open in the wake of the attack Friday despite their systems being inaccessible.
Friday's attack follows a wave of similar attacks on municipal government computer systems across the country.
In January, the St. Louis Public Library system was hit with ransomware, bringing the city's 16 branches to a halt. In lieu of paying the $35,000 demand, CNN Money reported
at the time, the library planned to wipe its 700-terminal computer system and reset it.
Last month, CNN affiliate WTAE reported
that a ransomware attack shut down government and police computer systems in Ohio's Licking County.