The FBI sent a warning to law enforcement and other public officials on Tuesday to be on guard for cyber attacks from hackers.
"Law enforcement personnel and public officials may be at an increased risk of cyber attacks," the FBI notice says. "These attacks can be precipitated by someone scanning networks or opening infected emails containing malicious attachments or links."
The hacking threat alert comes as police departments in several parts of the country are facing criticism and protests for mistreating or killing crime suspects.
In the most recent incident, a black man, Freddie Gray, died in the custody of the Baltimore police. The case has triggered demonstrations.
According to the notice, unspecified hacker groups have exploited open source and other publicly available information that identifies law enforcement personnel.
"Those targeted include law enforcement officers, their employers and their families," the notice says.
"With this in mind, officers and public officials should be aware of their online presence and exposure," the warning said. "For example, posting images wearing uniforms displaying name tags or listing their police department on social media sites can increase an officer's risk of being targeted or attacked."
Although not listed in the notice, the hacker group in question appears to be the Anonymous hacker collective, which is said by security analysts to have sophisticated cyber attack capabilities.
A Twitter feed associated with the group stated Wednesday, "Police officers of America we are watching you. Everyone is watching you. We suggest you stop murdering people in your custody."
Anonymous conducted cyber attacks against police in Oakland, Calif., in December to protest a grand jury decision not to charge white police officers in the deaths of black suspects in Ferguson, Missouri, and New York City.
In 2011, the group claimed it hacked some 70 local police department computer networks and, in at least one instance, disclosed data pertaining to an ongoing investigation. It posted some of the hacked data, which included emails, informant tips, and credit card numbers.
The FBI notice, issued through a unit called the Internet Crime Complaint Center, said law enforcement and public officials should be aware of how content posted on social media could be used against them during online attacks.
"The act of compiling and posting an individual's personal information without permission is known as doxing," the report said. "The personal information gathered from social media and other Web sites could include home addresses, phone numbers, email addresses, passwords, and any other information used to target an individual during a cyber attack. The information is then posted on information sharing web sites with details suggesting why the individual should be targeted."
In addition to law enforcement personnel, hackers appear to be targeting family members, using online personal information and photos from social media site.
"Another dangerous attack often used by criminals is known as ‘swatting,’" the FBI said. "This involves calling law enforcement authorities to report a hostage situation or other critical incident at the victim's residence, when there is no emergency situation."
The notice said that while eliminating exposure to hacking in the digital age is nearly impossible, law enforcement and other public officials can take steps to reduce risks.
These steps include using all privacy settings on social media sites and refraining from posting photos showing affiliation with law enforcement. Other measures include limiting personal postings on media sites and using caution in posting comments. Routine updating of software to patch security flaws is another suggestion.
The FBI also recommends conducting routine name searches online to see what information is publicly available. The Bureau also recommends using two-factor security for email.
"Closely monitor your credit and banking activity for fraudulent activity," the FBI said, adding that passwords should use a phrase of 15 characters or more and should be changed regularly to reduce the potential for compromise.
Hackers also use "social engineering" by deceiving targets through phone calls into providing confidential, personal, or sensitive information that can be used in later cyber attacks.
FBI spokeswoman Carol Cratty declined to say if Anonymous is the cause of the alert or whether the notice is related to recent incidents of police activity.
"The FBI periodically sends out these types of advisories as a service to our law enforcement and private sector partners," she said.