A policeman has been arrested after a student in California used the "gender swap" filter on Snapchat to pose as an underage girl.

The 40-year-old policeman was arrested after exchanging explicit messages with what he thought was a 16-year-old girl.

"Robert Edward Davies, an officer with the San Mateo Police Department, was arrested Thursday on suspicion of contacting a minor to commit a felony," San Jose Police Department said in a statement.

The student's revealed in an interview with NBC Bay Area, who only wanted to be identified by his first name, Ethan, said that his vigilant effort was sparked after a friend told him she was sexually assaulted as a child. He wanted to pose as an underage girl online to catch sexual predators.

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Police also revealed that the 20-year-old student took a selfie with Snapchat's new gender-swapping filter and created a 19-year-old female Tinder identity named "Esther."

"I believe he messaged me, 'Are you down to have some fun tonight?' and I decided to take advantage of it," said Ethan.

The policeman allegedly chatted about sexual activities with the student's fake profile despite being told "she" was underage. Ethan claimed Davies did not seem bothered by Esther's age.

The policeman allegedly asked for the communication to continue over Snapchat. "We started texting on there, and it got a lot more explicit," Ethan said in the interview.

The student turned his phone on airplane mode which stopped alerts to Davies that their conversations were being screenshotted.

Ethan sent the screenshotted conversations to Silicon Valley Crime Stoppers which led to an investigation and the arrest of Davies.

The policeman was charged with communicating with a minor to commit a felony and is now on paid leave from the police force.

Ethan revealed that he would not go undercover to catch predators after this experience. "I was just looking to get someone," he said. "He just turned out to be a cop."