FLINT -- Thieves have been picking away at the skeleton of one of the last buildings left standing at the mostly abandoned Buick City complex, making off with more than $100,000 in scrap metal and wire from just one decommissioned building. A General Motors spokesman and Genesee County Sheriff Robert Pickell confirmed that an investigation is underway, and Pickell said he has seven suspects in what appears to have been a carefully planned and executed criminal operation. Not only did burglars have to go onto the site, which is patrolled by security officers, and into a locked building that's known as Plant 36, they had to carry the scrap metal off the site without being noticed as well. "It appears to (have been) an ongoing criminal activity costing General Motors hundreds of thousands of dollars if not more," said Pickell, who declined further comment because the investigation — being done in conjunction with Flint police — is ongoing. The thefts are just an asterisk in the long history of the sprawling Buick City complex on Flint's North Side, where only a few hundred workers still produce parts for GM vehicles at what's called Powertrain Flint North. Much of the Buick City site has been decommissioned or demolished since vehicle production ended here a decade ago. The thefts apparently came from Plant 36, which had produced Buick engines until August of 2008. Built in 1951, Plant 36 is 1.13 million square feet and has produced both V8 and V6 powertrains — more than 30 million of them since opening. The Harbour Report recognized the plant as a leader in its segment for productivity on numerous occasions. Gerry Godin, a former Buick worker who writes about the history of Buick factories in Flint on his blog, "All Things Buick," said the fate of Plant 36 is just one of the sad images left behind at the 250-acre complex. "It's sad to see it all gone. All you see there now are (things like) 10-foot trees growing up through the parking lots," Godin said. "It's hard to look at." "I wish the place was still there" like it was, he said. "Everybody knew it was going to go down eventually because of the way it was run ... but everybody had it somewhere in their heads that some miracle would happen and it would keep going too." Godin said he's been stopped from taking pictures around the site by private security officers but said securing as many acres as there are on the site would be a daunting task. UAW Local 599 President Bill Jordan, who represents remaining workers at Flint North, said Plant 36, located on the northern-most part of the site, is massive and hard to watch all at once. "You're talking about a building that's a mile around and not really occupied," Jordan said. GM spokesman Tom Wickham said the company is aware of the thefts and officials are cooperating with Pickell ad Flint police in the criminal investigation. "We are cooperating with the local authorities as it relates to theft on the property," Wickham said, "We're doing whatever we can to address the situation." GM won't confirm the dollar value of the scrap metals taken from the site or discuss past reports of unauthorized people on the site or thefts from unoccupied buildings. Theft from abandoned plants in Flint isn't unheard of, however, and copper and scrap-metal-seeking thieves have stripped many unoccupied Flint homes of copper wiring. Earlier this year, security guards watching the old Delphi complex at Davison Road and Dort Highway caught a man suspected of having stolen $15,000 worth of grade-A copper from the property over an eight-month period. In that case, thieves apparently timed their entry around security checks, entering a building through a hole in the wall. Production at three buildings that remain in use on the Buick site is expected to be completed by the end of 2010. Economic development officials have spent the past several years promoting the Buick City location as the site of a potential new inter-modal transportation hub for the distribution of goods from truck and rail.