ROOSEVELT — Investigators with the U.S. Bureau of Land Management and the Interior Department's Office of the Inspector General are being tight-lipped about their inquiry into alleged problems with equalizing valves used by two oil companies operating in the Uinta Basin.

For the past two months authorities have been looking into whether employees of Berry Petroleum Co. and Quinex Energy Corp. tampered with or improperly installed the equalizing valves, which ensure that extracted crude oil doesn't spill from storage tanks at well sites.

Local BLM officials have declined to speak about the investigation, referring calls for comment to the Inspector General's Office in Washington, D.C.

"There is an investigation ongoing," said Bill Stringer, field manager for the BLM's Vernal office. "I know enough to say I can't talk about it."

A call to Washington, D.C., also revealed very little about what specifically authorities are looking into.

"I can't say too much about it because it might have some criminal implications, but we were out there," said Roy Kime, a spokesman for the Inspector General's Office.

Equalizing valves are found at most well sites on a section of pipe that connects two storage tanks. When properly working, they allow crude oil from one tank to spill into the neighboring tank to prevent overflowing.

Tampering with the equalizing valves, or improperly installing them, could allow oil to continue to flow from one tank to another without accounting for the extra oil. That might change the volume in a storage tank after it's been measured for sale, but before it's pumped from the location into a tanker truck.

The investigation into the valves started, according to oil field sources who spoke on condition that they not be named, in May after BLM employees visiting a Berry well site noticed that oil was flowing from one storage tank into another despite the valve being in the closed position.

A closer inspection of the equalizing valve, one source said, revealed that only the outer housing was present; the internal components had been removed. The discovery led to a BLM inspection of well sites belonging to every oil company operating in the Uinta Basin.

Kime would not comment on the condition of the valves or on the breadth of the investigation.

In an e-mail, Berry Petroleum spokesman Todd A. Crabtree acknowledged that the company was answering questions from the BLM about "certain operational equipment" in its Brundage Canyon field in the Uinta Basin.