A San Jose police officer is being criminally investigated for handcuffing his stepdaughter’s 15-year-old boyfriend and pretending to arrest him for having sex with the 14-year-old girl.

Prosecutors are probing the incident — parts of which are caught on a smartphone video — to see whether the officer committed a criminal false imprisonment soon after he showed up at the house on his motorcycle and in full uniform late last month.

“Not a good thing that the person you had sex with is a cop’s daughter,” the officer sternly says, standing over the handcuffed boy in his family’s living room, according to the video. “The district attorney will probably file charges. “… A cop’s daughter is not somebody you mess around with. You’re stupid.”

The veteran motorcycle officer, who has since been placed on administrative leave, is saying he was simply trying to scare the boy straight, according to his lawyer.

But the teenager’s parents said they encountered an angry cop abusing his authority.

“He came over in full uniform and parked his motorcycle in front of our house; he didn’t come over in jeans and shirt,” said the boy’s mother. “He didn’t come over as a parent. He came over as somebody who had the authority to do whatever he wanted with us.”

Compounding their disbelief, when the boy’s parents reported what happened to San Jose police internal affairs, their son was later arrested for unlawful sexual intercourse. The Mercury News is not naming the officer nor the boy’s parents to protect the identities of their children.

District Attorney’s Office spokeswoman Amy Cornell confirmed that prosecutors are reviewing the case and have sent it back to the police for more information.

The conflict began Aug. 30, apparently soon after the officer found out that his teenage daughter had sex with her boyfriend. The two had dated since middle school, giving each other gifts that made both sets of parents concerned, according to the boy’s parents. He bought her a pair of Uggs. She gave him $100 in cash. Then the boy recently showed up at a house where the girl was baby-sitting. They became intimate that night.

When the officer found out, he rode his motorcycle at the end of his shift straight from work to the boy’s house, a few blocks from his own home, according to the boy’s parents.

The officer knocked on the door, and the boy’s mother asked him why he was there. According to the boy’s parents, the officer said he had found their address after running a check on their license plate — which, if true, could be illegal. And then he dropped the bombshell.

“I am going to arrest your son for sexual assault,” the officer said, according to the parents.

When the boy got home soon afterward, the officer, according to the parents, began berating the teenager, telling him he had warned him to stay away from his daughter.

Then, he turned the boy around and placed handcuffs on him. The boy’s stepfather video-recorded some of the encounter on his BlackBerry and provided it to the Mercury News.

Finally, the officer unlocked the handcuffs and brought the parents aside.

He was not going to arrest their son after all, he told them. He wanted to scare him. He handed them an arrest card that he suggested they put up on the refrigerator to make sure the boy didn’t forget.

The officer’s story, as recounted by his lawyer Terry Bowman, jibes with the parents’ on some levels. But he says that he never pretended to arrest the young man. He said the parents told him their son was out of control.

“He is doing (the boy’s parents) a favor by attempting to lecture and educate the young man so he doesn’t continue to go down the wrong path,” Bowman said. “This is two sets of parents navigating the challenges of raising teenage children. The officer’s goal was to help and engage in problem solving. Turning this into the blame game does not help the teens. In fact, it sets a poor example for taking personal responsibility.”

Bowman said the parents thanked the officer after the encounter and shook his hand.

The stepfather said he may have done that in an effort to get the officer to leave, but the mother said she didn’t recall doing that. She said she went to a Walgreens parking lot and cried.

Police said there is no written duty manual section that prohibits an officer from handling a case that involves a family member or friend.

There is a city policy that says employees must be impartial in their duties.

“We do not generally have our officers investigate cases when they have a personal conflict,” said police Sgt. Ronnie Lopez, a department spokesman. “It’s common sense.”

Police sources say it is not uncommon for officers to stop, search and sometimes handcuff youths if the parents ask them to as part of a “scared straight” tactic. But the parents of this boy say they gave no such permission and were shocked at what the officer did.

Before they made a complaint to police, the parents of the boy say they agonized.

“He abused his power, but we didn’t know what to do,” his stepfather said. “I don’t want any kind of payback. And if they go forward with investigating this officer, then we were worried they might go after (my son) with a charge of sex assault.”

On Saturday, a San Jose detective called the boy’s mother and asked to interview him. Afterward, the boy was cited for unlawful sexual intercourse — a misdemeanor citation for two minors of a similar age who have consensual sex. Sources told the Mercury News that the officer’s daughter was also cited.

The District Attorney’s Office rarely prosecutes minors on such charges, generally rejecting cases that don’t involve coercion.

Contact Sean Webby at 408-920-5003.