"The FBI has observed malicious actors targeting healthcare related systems, perhaps for the purpose of obtaining Protected Healthcare Information (PHI) and/or Personally Identifiable Information (PII)," the agency said in a "Flash" alert obtained by Reuters on Wednesday.
The FBI and Department of Homeland Security periodically release alerts to provide U.S. businesses with technical details and other information they can use to either prevent or identify cyber attacks. Such reports are typically only issued to businesses and not distributed to the general public.
The FBI has been concerned about healthcare providers for several months. In April, it warned the industry that its systems were lax compared with other sectors, making it vulnerable to hackers looking to access bank accounts or obtain prescriptions.
The agency has also reached out to other industries, including a warning to retailers in January alerting them to expect more credit card breaches in the wake of last year's attack on Target Corp.
The recent alert to healthcare companies did not identify any specific victims targeted by hackers. An agency spokesman declined to comment on the document.
Community Health, the No. 2 U.S. publicly traded hospital operator, disclosed the attack on Monday, saying stolen data included patient names, addresses, birth dates and Social Security numbers.
The healthcare company has said little about how its network was attacked.
David Kennedy, an expert in healthcare security, said he has learned the hackers broke into the company's computer system using a piece of networking equipment that had not been patched to fix the "Heartbleed" Internet bug. The break-in was the first known large-scale cyber attack to exploit that vulnerability.
Kennedy, who is chief executive of TrustedSec LLC, said multiple people familiar with the investigation told him hackers exploited the bug in a piece of Juniper Networks Inc equipment to obtain employee credentials and access the company's network. Once in, they hacked their way into a database containing Social Security numbers and other records.
Juniper spokeswoman Danielle Hamel declined to comment on the breach, but said her company issued patches in April to protect customers against Heartbleed.
Community Health spokeswoman Tomi Galin did not respond to requests for comment on Heartbleed.
(Reporting by Jim Finkle. Editing by Andre Grenon)