Ethan told the station: "I believe he messaged me, 'Are you down to have some fun tonight?' and I made a decision to take advantage of it".
Ethan, who did not disclose his last name in fears of retaliation, told NBC Bay Area that he was looking to catch pedophiles because his friend had been sexually assaulted as a child.
"He just happened to be a cop", he said.More news: MPs may have run out of options to block no-deal Brexit
Robert Davies, 40, an officer with the San Mateo Police Department, was arrested June 6 and charged with contacting a minor to commit a felony, according to police.
Two days later, detectives with the San Jose Police Department launched a criminal investigation.
In 2018, A CT man was caught by a group called POPSquad, which lured him to meet up with a 14-year-old boy they had posed as on a dating app and posted a video of the encounter onto their Facebook page of more than 17,000 followers. Ethan said the pair exchanged messages for over 12 hours, with screengrabs of their conversations sent to Crime Stoppers. There, he told the police officer he was 16 years old and asked if it bothered him. Then he shared them with police.
The screenshots, which Ethan described as "explicit", lead to Davies' arrest last week on suspicion of discussing sexual activity with a minor on social media.More news: 2nd Ebola patient dies in Uganda, health ministry says
'Whether or not the person is 16 is irrelevant, if the suspect believes it's a 16-year-old on the other end, the suspect should have terminated that conversation - specifically when it talks about engaging in sexual activity, ' Sgt. Enrique Garcia told KTVU.
A male university student used Snapchat's new "gender switch" filter to take down a police officer who was allegedly attempting to meet with an underage girl.
In a statement on the San Mateo Police Department's Facebook page, police chief Susan Manheimer said the conduct, if true, "is an affront to the tenets of our department and our profession as a whole".
Last week, the San Mateo Police Department released a statement in response to Davies' arrest, explaining he was off-duty and in another country when the alleged crimes took place.More news: Avengers: Endgame star Chris Hemsworth REVEALS his freaky first job; Watch
'Through the messages, on objective I would just get these little bits of information about him so it would be easier to track him down, ' Ethan told NBC.