Law enforcement officers in the United States are increasingly buying professional liability insurance policies amid worries they may be sued for their on-duty actions, the Fraternal Order of Police, the biggest U.S. police union, told Reuters.

Between July 2014 and July 2015, the number of members who bought the union's liability insurance jumped 15 percent, according to data from the FOP released this week and shared exclusively with Reuters.

In previous years, liability insurance purchases grew only between one to three percent, said Jim Pasco, executive director of the FOP, which represents more than 330,000 officers.

The insurance would help cover legal fees, but would not protect against criminal prosecution.

The sharp jump comes amid a national debate over whether law enforcement should be under more surveillance and scrutiny, especially in light of a wave of publicized cases where deadly force was used against unarmed civilians, many of them minorities.

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"In an already litigious society, the likelihood of a police officer being sued or charged, often falsely, grows by the day. Officers are increasingly aware of the need to be protected and joining the FOP legal defense plan in growing numbers," said Pasco, whose union sells insurance for $265 a year.

Civil rights groups such as Black Lives Matter and the Obama administration have called for increased accountability for officers, including the use of body cameras and community oversight.

Still, some in law enforcement, including FBI Director James Comey, have said that the increased scrutiny has made officers feel antagonized and afraid to do their jobs.

"The environment has become increasingly volatile towards law enforcement in general," said Jonathan Adler, a member and past president of the Federal Law Enforcement Officers Association, where insurance purchases grew 10 percent last year.

Several high-profile police-involved shootings between July 2014 and July 2015, the period the union studied, led to civil and criminal suits being filed against police.

Officers sued in wrongful death lawsuits include Ferguson, Missouri officer Darren Wilson, who shot unarmed teen Michael Brown in August 2014, and Cleveland officers Frank Garmback and Timothy Loehmann, the officers who encountered Tamir Rice, 12, who was shot while carrying a toy gun in November 2014.

The insurance also covers officers charged in criminal cases like Michael Slager, the officer charged in the murder of Walter Scott in North Charleston, South Carolina in April 2015.

The Justice Department declined to comment on the rise in officer liability insurance.