"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."

Company representatives could not be reached for comment on the incident.

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.

The Associated Press reported that the Federal Bureau of Investigation was investigating the attack on MedStar. FBI officials could not be reached for comment.