FORT WORTH -- A Fort Worth police officer arrested last week on accusations he spanked the rear of a woman he caught engaging in sexual conduct in a car in Oakhurst Park was charged with official oppression.

If convicted of that charge, a Class A misdemeanor, officer Craig Murrah faces up to one year in jail and a $4,000 fine.

Prosecutor Kurt Stallings confirmed Thursday that Murrah had been officially charged but said the case will also be presented to a grand jury.

Police have said that at about 1 a.m. on June 22, Murrah found a couple engaging in sexual conduct in a car at the park at 2300 Daisy Lane in north Fort Worth. The officer ordered the couple out of the car and placed the 18-year-old woman, nude from the waist down, in the back of a patrol car. At some point during the investigation, Murrah allegedly spanked the woman on the rear, police have said.

The woman said the officer let them go with only a warning.

When the woman told her boyfriend what the officer had done, he called 911. Police conducted an internal investigation, and a warrant was issued for Murrah's arrest.

Wes Ball, Murrah's defense attorney, said Thursday that they will wait to see all the details but said they intend to plead not guilty. He also pointed out that the couple, by their own admission, was breaking the law by having sexual contact in public.

"They were violating the law, and, as far as I understand it, officer Murrah has provided 6 1/2 years of good service," Ball said. "This is a grave thing for him, his future and his career. He has asked me to help him, and that is what I intend to do."