SAN FRANCISCO (KGO) -- San Francisco police are trying to figure out who's responsible for chaining baseball bats to street poles across the city.Police in the Parkside neighborhood found one of those bats. It's a scene that was repeated more than a dozen times Thursday. In some cases, the bats also had nails protruding from them.The bat at the Parkside location looked like a pipe bomb. Officers cleared the area and brought in the bomb squad. It's a mystery as to why these weapon-like items were placed across the city."We thought it was possibly an explosive device. As you can see we evacuated the area," said San Francisco police Lt. Nick Rainsford.Someone called San Francisco police about the baseball bat chained to a pole at 19th Avenue and Buckingham Way. It's sawed-off with a square piece of metal attached."This was kind of an anonymous call.It was a little different from the rest," Rainsford said.More than 15 bats chained to poles have been located across the city. Gerrie Burke snapped a picture of a bat near Mission and Rolph streets earlier in the day."This is a little stranger than normal," San Francisco resident Gerrie Burke said. "So, we called the cops just in case and like two cars showed up immediately and said that was the eleventh one of the day."Many of the bats looked alike."There were bolts in this thing. They had drill holes in it. They knew what they were doing. Whoever did this put it together, this was not put together in an hour," Burke said.The bomb squad X-rayed and cleared the bat along 19th Avenue. Police have no idea who's placing the would-be weapons, or why."Art project, freak people out, or a promotion or something," Burke said.Online forums show people how to make similar bats. They're used in movies and video games featuring zombies. Real weapon or prop is anyone's guess."It's not a really funny joke it's a big waste of police resources," Rainsford said.The bats are being taken as evidence. Barring the zombie apocalypse, the person responsible could face charges.