A Massachusetts State Police sergeant has filed suit against the department, saying she faced retaliation after reporting a trooper for keeping pornographic images on a police hard drive and having sex in a hotel while on duty.

Sgt. Kathryn Downey, a 13-year veteran of the state police force, took swift action by reporting the alleged misconduct by Trooper Earl Johnson, her ex-boyfriend, according to the lawsuit filed in Suffolk Superior Court.

But after notifying superiors of Johnson's extracurricular activities, Johnson allegedly filed a retaliatory complaint against her for a year-old prank -- costing her an assignment as a training instructor and leaving a black mark on her disciplinary record.

"After having lawfully complaint about Trooper Johnson's sexually transgressive behavior in the workplace, Trooper Johnson acted maliciously and intentionally in retaliating against Downey by filing a complaint against her containing numerous, easily proven false statements, with the malevolent intent to destroy her career," the suit claims.

Downey, who is being represented by Attorney Leonard Kesten, is accusing Johnson and Massachusetts State Police of sexual discrimination, whistleblower violations and retaliation. She was a trooper at the time and was later promoted to sergeant.

State police spokesman David Procopio wrote in a statement that Downey was appropriately disciplined for the prank, which involved her firing a dummy Taser charge at Johnson.

"The plaintiff committed a serious violation of rules governing use of department weapons. Internal charges against both the plaintiff and the trooper named in the suit were sustained by the Department: Against Trooper Downey for firing an electronic control weapon at Trooper Johnson, and against Trooper Johnson for violating the Department's information technology policy," Procopio wrote. "Both of them received appropriate disciplinary action and both were removed from their Academy positions."

The suit is the latest in a series of legal controversies to hit the department, including ongoing investigations over alleged overtime abuse and lawsuits by two troopers, also represented by Kesten, who claim they faced retaliation for refusing to a soften an arrest report for a judge's daughter.

In 2015, after about two years working as an adjunct instructor at the Massachusetts State Police Training Academy, Downey was assigned to teach defensive tactics. The next year, she and two other troopers -- Steven Wohlgemouth and David Lahair -- decided to play a prank on Johnson, who she was dating at the time.

Wolhgemouth had created a dummy Taser cartridge that looked like a live cartridge, which Downey shot at Johnson as a joke, according to the suit.

"Trooper Johnson's initial reaction was to boast that the Taser could not hurt him because he was 'Superman.' Downey then explained that it was a prank and that Wholgemouth had created a fake black cartridge," the suit claims. "Everyone present laughed at the prank."

No complaints were filed about the fake tasing, until over a year later -- when Downey had reported Johnson for sexual improprieties on the job, according to the suit.

In June 2016, Downey was looking for training materials on Johnson's portable hard drive when she allegedly found multiple folders that contained "sexually suggestive photographs and videos of naked women," women performing sex acts and pornographic images of Johnson himself. Another folder allegedly contained photographs of Downey, her friends and family.

Downey also discovered that Johnson had checked into a hotel while on-duty, and confronted him in a phone conversation.

"Trooper Johnson stated to Downey that he was sex addict and that he was 'out of control,' " the suit claims. "He also indicated that he had checked into a hotel to have sex with a woman while on duty."

Downey reported that information to her superiors, and in March of 2017 Johnson was transferred out of the academy and received "some form of discipline," according to the suit.

But Johnson was allegedly angry over Downey's report and the next month he lodged a complaint against her over the 2016 Taser prank. He also allegedly contacted the Taser manufacturer and attempted to have her training license revoked.

That fall, false rumors began to spread that Downey had a sexual relationship with the supervisor who had taken her complaint against Johnson, according to the suit. She filed a complaint with then-Col. Richard McKeon, asking for an investigation of the rumors.

But she was instead transferred out of the training academy, and was given a letter of reprimand for the prank, the suit claims. State police also concluded that Johnson had not retaliated against her, despite both Wholgemouth and Lahair telling investigators that the Taser prank was a harmless joke, according to the suit.

"One of the Troopers, when asked, stated that the reason that Trooper Johnson filed his complaint against Downey was 'retribution,' " the suit claims.

The punishment against her was discriminatory and the result of retaliation, the suit claims, noting that she and Johnson received similar discipline despite his allegedly conduct being a much more serious violation.

And while Downey was issued a letter of reprimand, Wohlgemouth -- who created the prank Taser cartridge -- was not disciplined, according to the suit.

"To be clear, Downey does not believe that Trooper Wohlgemouth should be disciplined; however, the fact is that she is being disciplined and a male Trooper who participated in the prank with her is not," the suit says.