ST. LOUIS (AP) -- A backup warning system did not prevent the Taum Sauk reservoir collapse because Ameren Corp. employees effectively disabled a set of alarms by readjusting them to keep the hydroelectric plant running at full capacity, according to a newly released report.

The so-called "Warrick Probes" were set along the top of the mountaintop reservoir, and were designed to shut the plant down and prevent an overflow if water ever reached the probes.

But Ameren employees told Missouri Highway Patrol investigators that the probes were raised up so high that water never touched them the morning of Dec. 14, 2005 when the reservoir overfilled and collapsed, causing one of the worst man-made disasters in Missouri history.

The probes might have been raised as late as Oct. 2005, when Ameren employees knew the reservoir was in danger of overflowing because separate water-level gauges were broken, according to the report.

An unknown Ameren employee removed the probes immediately after the reservoir collapse. The probes were stacked in a bucket at the bottom of the reservoir by the time Ameren allowed safety inspectors from the Missouri Department of Natural Resources to examine the facility, according to James Alexander, DNR's head of dam safety.

Now investigators might never know for certain how high the probes were raised, or if they were even operational at the time of the collapse, Alexander said.

"We don't know because they went up there and jacked with the evidence before we could find out," Alexander said. "There was no way that anybody could tell just how bad it was -- what level it had been set and just how negligent they had been."

The patrol investigated the collapse for a year and a half, interviewing everyone from Ameren executives to the plant's security guards and superintendent.

But the patrol's 2,000-page report is not conclusive. It states that Ameren never supplied all the e-mails and documents that investigators requested. It also says that no one ever acknowledged altering the probes, even though Ameren employees acknowledged only a handful of people knew how to change the devices.

"In the course of all interviews, no one has admitted to making a manual adjustment to the probes, which caused the incorrect reading," the report says. "In short, no one has been identified as having actually 'pulled up' or moved the location of the probes along the inside of the reservoir walls."

Ameren spokeswoman Susan Gallagher and spokesman Tim Fox did not return several messages left over five days seeking comment for this story.

Missouri Attorney General Jay Nixon said he will not file criminal charges against Ameren over the collapse. He said the patrol's report did not identify any suspects in the case.

Nixon spokesman Scott Holste said Nixon made the decision not to prosecute Ameren because the patrol report did not identify a suspect. Nixon's own criminal investigators did not look into the matter because the patrol had authority on the case, Holste said.

"We're not going to go in and have our own people do duplicative efforts in trying to get information," Holste said. "The highway patrol was the investigative body."

On the morning of Dec. 14, the Warrick probes never indicated the Taum Sauk reservoir was overflowing. Water poured over the top concrete wall of the basin, eroding the outside of the reservoir's earthen wall. The concrete slabs eventually gave way, increasing the flow of water and causing a total failure of the structure.

The reservoir breach unleashed 1.3 billion gallons of water that swept over Johnson's Shut-Ins State Park, injuring the park superintendent's family and devastating much of the popular tourist destination that remains closed.

The Warrick Probes were critical to keeping the Taum Sauk reservoir safe. Powerful pumps filled the 55-acre-wide basin each night and no one was on site to make sure the basin did not overfill.

The Warrick Probes were originally set at two levels. Probes set at the "Hi" level would warn operators when water was close to the top of the reservoir walls. Above these probes, the final set of "Hi-Hi" gauges would shut down pumps feeding the reservoir if water touched them.

Interviews with Ameren employees by The Associated Press over the past year have confirmed Ameren's practice of filling the reservoir as high as possible because every foot of water represented profitable electricity generation. That sentiment was also made clear in the patrol's report.

Taum Sauk Superintendent Richard Cooper told patrol investigators he felt pressure from his bosses after he ordered the water level to be slightly lowered to prevent an overflow in October 2005.

"Cooper stated that he had people above him and below him that wanted to know what was going on. Since the upper reservoir was set two feet lower, that was resulting in producing less mega watts (sic) of electricity," the report said.

The warning probes put a glitch in power production by producing several "nuisance alarms" that were caused by "wave action" when strong winds led water to hit the probes, according to the report.

Ameren Vice President Mark Birk told DNR investigators that Ameren reprogrammed the alarms so that both the "Hi" and "Hi-Hi" sets needed to be submerged in water for a full minute before the pumps were automatically shut down, the report stated.

In addition to the reprogramming, someone moved the "Hi-Hi" probes at least several inches up the side of the reservoir wall. It's unclear who removed the probes immediately after the reservoir collapse.

Birk told DNR engineer Paul Simon that an Ameren employee removed the probes, according to Simon's investigation, which is included in the patrol report. Simon asked Birk who removed the probes after the collapse, and was told that Ameren did not know.

"Mark Birk (Ameren) told us that the HI and HIHI probes were pulled out and tested by one of their engineers on the day of the event, in the engineers (sic) excitement," Simon's report said.

Ameren declined to make Birk available to comment for this story.