TORONTO -- A six-year investigation into alleged wrongdoing by senior staff at Ontario's air ambulance service was unable to gather enough evidence to support criminal charges, provincial police announced Thursday, saying they were hampered by a lack of co-operation from key individuals and inadequate business records.

The Ontario Provincial Police investigation of Ornge began in 2012 at the request of the Ministry of Health and involved a multimillion-dollar helicopter deal and alleged kickbacks involving the top employees.

"The lack of accountability and transparency in the business records, the variance in perspectives and the lack of co-operation of key persons involved collectively has prevented an evidence based finding from being made," said the OPP's investigative summary, a 94-page document released Thursday with dozens of pages blacked out.

Barring further information surfacing or the co-operation of witnesses, the investigation is over, the police service said.

"Reasonable grounds do not exist that a criminal offence occurred," the summary said. "Further, no further investigative steps exist that will assist in the gathering of independent and reliable evidence to inform and allow investigators to make an evidence based decision as to whether reasonable grounds exist."

Police say 58 people in Canada, the U.S. and Italy were interviewed during the probe that cost almost $250,000.

At the time the investigation began, Ornge was under a cloud of controversy over high executive salaries, and questionable spending and business practices.

Then health minister Deb Matthews cleaned house at the agency in January 2012, replacing CEO Chris Mazza -- who was paid $1.4 million a year -- and the entire board of directors. ORNGE was also ordered to shut down its for-profit companies.

At the center of the police investigation was a $144 million deal to buy a dozen helicopters from AgustaWestland Aerospace, a U.S. subsidiary of a company heaquartered in Italy, and $4.7 million in alleged kickbacks returned to the Ornge's for-profit side.

Ornge had grown to become a conglomerate of 20 interrelated not-for-profit and for-profit companies, only eight of which had any employees, a 2013 report to a committee of the legislature said. The for-profits were ultimately controlled by certain board directors and Mazza. Almost all the money that flowed to them came from government funds.

Each board within the conglomerate was controlled by a common group of individuals, the report said. Many of the directors were also Ornge officers and employees. However, shoddy record-keeping -- or no records at all -- prevented auditors from figuring out where much of the money actually went.

Police considered a number of possible charges but ultimately decided they could not proceed based on a lack of evidence, the summary said, adding that significant questions have been left unanswered.

"OPP Investigators considered a potential criminal offence of misappropriation of funds involving alleged kickbacks received by Ornge in the helicopter purchase," the summary said. "This offence was not supported by evidence resulting from the investigation."

Ornge said it had been "respectful" of the investigative process and co-operated fully with the OPP.

"Everyone at Ornge is focused solely on ensuring the best possible medical transport services for the people of Ontario," it said in a statement. "We are well into an era of transparency, accountability and operational integrity."

Health Minister Eric Hoskins said "significant progress" has been made since 2012 in Ornge's service delivery and the government's oversight of the agency.

"The ministry has every confidence the leadership at Ornge will operate responsibly, meet and/or exceed aviation safety standards, provide high quality patient care and cost efficient services, and fully co-operate with the ministry," Hoskins said in a statement.

Progressive Conservative health critic Jeff Yurek said the OPP was "stymied" and nothing police can do will return money the agency wasted.

"The auditor general has confirmed that millions of tax dollars were wasted because this government let Ornge executives run wild," he said.