MedStar Health, one of the biggest medical providers in the Washington, DC, area, said on Monday it shut down some computer systems after discovering a computer virus.
"Networks temporarily shut down to prevent virus spread," MedStar, the operator of 10 hospitals in Washington and Maryland, said on Twitter. "We have no evidence of compromised information. All facilities remain open."
The Federal Bureau of Investigation said it was aware of the incident and investigating. Company representatives could not be reached for comment.
Computer systems have been down across the MedStar network for a few hours, two employees told Reuters, adding that they had received no indication yet when service would be restored.
Employees are recording patient data by hand and are unable to process copayments at this time, an employee at the MedStar Promptcare in the Adams Morgan neighbourhood of Washington said.
It was not immediately clear what type of virus had infected the company's network. Yet its discovery comes at a time of unprecedented concern about security of healthcare providers in the wake of several attacks on U.S. hospitals by cyber extortionists.
The attacks used software known as ransomware, malicious software that encrypts data and demands that users pay to get it unlocked.
Security blogger Brian Krebs last week reported that Henderson, Kentucky-based Methodist Hospital declared an "internal state of emergency after falling victim to a ransomware attack. Last month, Hollywood Presbyterian Hospital in Los Angeles paid a ransom of $17,000 to regain access to its systems after a similar attack.
The Washington Post quoted MedStar spokeswoman Ann Nickels as saying the company was working with "IT and cyber-security partners" as it assessed and responded to the situation.