DENVER -- Denver city auditor Dennis Gallagher said he was angered after an Aurora fire vehicle was spotted gassing up at a fleet maintenance facility for Denver.

“If we can’t get a good explanation, I think heads should roll," Gallagher told 7NEWS, adding his office started investigating when the complaint was made earlier in the day.

“One of our contacts observed an Aurora fire vehicle coming up and pumping away with the gas,” Gallagher said.

When Denver public works managers had no knowledge of an arrangement at the fuel depot between the two cities, Gallagher sent a letter demanding answers to Denver’s fire chief and local media.

In the letter, Gallagher said, he was “distressed” about an apparent, “informal off-the-books arrangement with the city of Aurora.”

Denver fire chief Nick Nuanes, who said he received the letter Tuesday afternoon, told 7NEWS an agreement is in place, and strongly denied any allegation of an off-the-books arrangement.

Nuanes said an intergovernmental agreement was signed in 1986 for the combined fire training facility with Aurora. Three years ago, an Aurora training officer asked if fuel could be purchased for a few vehicles kept on site, he explained.

“We worked out an agreement with (Aurora,) they pay for every penny of fuel they use,” Nuanes said. Aurora fire officials agreed.

“On a monthly basis we are invoiced by the city of Denver and we pay that invoice,” said Allen Robnett, a spokesman for the Aurora Fire Department.

Robnett added the training facility, called the Rocky Mountain Fire Academy, has "been very useful" to both cities and "saved taxpayers."

Gallagher said his staff could not find any contract outlining the fuel arrangement, or how accounting is done.

“We want to know how long this has been going on, how many cars have come through,” Gallagher. “It seems like a terrible lack of control.”

Chief Nuanes said he will cooperate fully with the auditor, and is willing to modify current practices.

“If we failed to have a separate agreement just for that fuel, well then shame on us. But there was never any secret deal that was done,” Nuanes said.

He provided 7NEWS with information about how much gas was used by Aurora recently. Just over $1,200 was billed last year for diesel fuel. The year prior, just over $900 was collected.