SAN FRANCISCO (CBS SF) — Dirt bike riders have been tearing through the streets of San Francisco for months, putting people’s lives in danger. Cops have had a really tough time catching them because the chase is simply too dangerous.

But thanks to KPIX 5’s Betty Yu, police now have the information they need.

You hear them before you see them, with a roar of engines in the night. It’s a group of motorcycle riders KPIX 5 has learned is known as Bay Area Supermoto, or BASM.

Videos they post on the internet show them riding in a pack down busy streets, running stop signs, doing wheelies, driving their motorbikes on sidewalks and up and down stairs, mostly in San Francisco.

“They meet at the marina, at the yacht club, usually Friday night,” one former rider told us. He doesn’t want to be identified, for fear of retaliation. “It’s almost a sport. It is a sport. Its how many dangerous maneuvers can you pull, how good are you at riding and not wrecking your bike into these cars and things that are around you.

But what really bothers him: He says the leader of the group is a law enforcement officer. “He would make comments like he is not too worried about the police because he is one,” said the former member.

He gave us a name: Zack Schlief, also known as Zack Douglas. On Zack’s Facebook page we found a photo of him posing by a Marin County sheriff’s squad car. We also spotted a familiar-looking motorcycle.

So could that be Deputy Schlief doing wheelies on the Golden Gate Bridge? Or speeding down the Embarcadero among a pack of riders, even tapping his head to give a signal that police are ahead?

The Marin County Sheriff’s Department confirms Zack Schlief is a deputy there. They tell us as a result of the information we gave them they are conducting an internal investigation, but can’t talk about it to protect the privacy rights of the public safety officer involved.

But San Francisco police had plenty to say. “It puts everyone in danger,” said Officer Carlos Manfredi. He says the motorcycle group has been on SFPD’s radar for months. “They are riding without any regard for public safety. They’re doing wheelies, performing stunts throughout the city. It’s all fun and games until someone gets hurt,” he said.

He was shocked to learn that the apparent leader of the group is a fellow law enforcement officer. “We took an oath, to serve and protect, and that oath doesn’t stop when our shift ends,” he said.

So what does Zack have to say? We were able to catch up with him as he came back from a daylong BASM ride. “We have seen you guys riding around San Francisco doing some crazy stunts, is there any truth to that?” we asked him. “No, that’s not what I do. I mean I ride with the group, but I am not doing anything like that.”

Betty: “We’ve heard that BASM, you being the leader of it.”

Zack: “I am not the leader. I am definitely not the leader of it.”

But we spotted him just hours before, giving instructions to newbies in the group. “Everyone new welcome, if you have never been on big rides before, it’s a lot of fun. When you are the one that is leading the ride, like me and him, it’s like f***ing herding cats,” we heard him say.

Betty: “We also know that you are with law enforcement. Is that correct?”

Zack: “No that’s not true.”

Betty: “It just seems like a conflict of interest knowing that you are a law enforcement official.”

Zack: (interrupting) “I am not doing any stunts I am sorry I am not talking to you.”

Betty: “There is video of you doing such things. That is why we just wanted to ask you a few questions because as a law enforcement officer by day, it seems like a conflict of interest to be doing this kind of stuff when you are outside the job.”

Zack: “Please do not talk to me please!”

We never got much further. After that Zack jumped on his bike and took off.

After we contacted the Marin County Sheriff’s Department Bay Area Supermoto changed its Facebook page and Instagram account to private. Zack Schlieff’s Facebook page is also down.