HARRISBURG — A witness told grand jurors investigating an alleged “pay-to-play” culture at the Pennsylvania Turnpike Commission that the agency, awash in toll-payers’ money, is like the Mesopotamia River.

“All of the animals come and drink there,” the grand jury report quoted the unnamed witness as saying. “If you were an engineering firm or a law firm or a consulting firm or whatever, you go to the turnpike because that is where the money is.”

On Wednesday, a former state senator, three former top Pennsylvania Turnpike Commission officials and two businessmen were charged in a sweeping investigation that prosecutors said involved a long-running scheme in which contract-hungry vendors gave lavish gifts and political campaign contributions to improve their chances of landing lucrative contracts.

The group of former state officials used their extraordinary power over multimillion-dollar turnpike commission contracts both to enrich themselves, rig bids for favored businessmen and help sway political campaigns, Attorney General Kathleen Kane said. The largely-secret investigation that came to light Wednesday dates back to 2009 and involved hundreds of witnesses to crack a political and corrupt culture, she said.

“Evidence of secret gifts of cash, travel and entertainment and the payment of substantial political contributions to public officials and political organizations by private turnpike vendors and their consultants demonstrates that the turnpike operates under a pay-to-play system that is illegal and corrupt,” Kane said.

The public, she said, “has lost untold millions of dollars” in inflated contracts, and she added that the “greatest improper influence” involved the turnpike’s procurement process.

Charges were filed against retired state Senate Democratic floor leader Bob Mellow, currently jailed in a federal prison in South Carolina on unrelated corruption charges. Mellow, 70, was accused of helping supporters and contributors get business from the turnpike and pressuring turnpike staff to support him politically and raise campaign funds.

Also charged were former turnpike chief executive Joseph Brimmeier, 64, and former turnpike chairman Mitchell Rubin, 61.

The investigation is continuing, but is restricted by an eight-year statute of limitations, Kane said.

Turnpike vendors named by the grand jury gave hundreds of thousands of dollars collectively in political campaign donations, while winning tens of millions of dollars in contracts over the past decade. Recipients of the campaign donations included former Philadelphia state Sen. Vincent J. Fumo, who is in federal prison on unrelated corruption charges, and former Gov. Ed Rendell.

The vendors also paid for gifts and entertainment worth tens of thousands of dollars, including trips to Paris and Vienna, tickets to professional baseball games, golf outings and expensive meals, some of which was not reported as required by the law, Kane said.

Business executives, including the two businessmen charged in the case, who participated in the pay-to-play system were rewarded with multimillion-dollar contracts, authorities said.

“The reason they made these contributions and provided these gifts (is) because they knew that was the way they would get these contracts,” said state police Commissioner Frank Noonan.

Companies identified by the grand jury, but whose executives were not charged with wrongdoing, included engineering firms, transportation planning and design firms and a major bond underwriter.