DENVER -- Four United Airlines employees allegedly bilked the company out of $136,000 by stealing blank passenger refund checks at Denver International Airport and splitting the money with other workers, a Denver grand jury said in indictments returned Thursday.

Eleven people were named in the indictment and investigators suspect even more people were involved.

"We feel the amount taken was substantially more than $136,000, as is often the case in any kind of check-related and fraud-related enterprise like this," prosecutor Phil Parrott said. "We feel we often only get the tip of the iceberg."

More than 400 refund checks were examined and each person named on the checks had to be contacted to determine whether they were given the check legitimately.

As a result, the investigators concentrated on four groups that appeared to have stolen and cashed the bulk of the checks, Parrott said. He said that each acted independently but was headed by a United employee and apparently was aware of the other groups' activities.

The 18-month probe focused on checks that the airline issues for hotel or food to passengers who are bumped or whose flights are delayed.

Two of the four employees, all former customer service representatives, were indicted on charges of racketeering and conspiracy.

The pair, Herbie Atencio, 33, and Sean Mooar, 34, allegedly recruited a large number of confederates. The coconspirators were trained by the pair on how to forge and cash the checks, the indictments allege.

The members of each group would then split the money and used it for entertainment, travel, property and living quarters, the indictments said.

Two other United employees, Joni Pippa, 47, and Todd A. Klein, 42, also are suspected of stealing the checks from the customer service counter, but they weren't charged with racketeering.

All four face counts of theft and forgery.

The thefts allegedly occurred during a nine-month period in 1999 and 2000.

Parrott said the probe started in the spring of 2000 when United officials became suspicious and turned over the results of an internal investigation to Denver police and the district attorney's office.

United spokesman Joe Hopkins said three of the four workers indicted no longer work for the airline. The fourth is now based in Orlando.

"We are extremely pleased with the announcement of the indictments against these individuals," Hopkins said in a released statement. United Airlines will not tolerate fraudulent activity by anyone, including our own employees. We would like to thank the District Attorney's Office for all of their efforts in pursuing these indictments."

Also indicted Thursday were William Cooper, 31; Jonathan Inge, 36; Lucas Wangerin, 21; Nicole Sisco, 21; David Cherry, 40; Donovan Pacheco, 30; and Fabio Moura, 32. Some have agreed to turn themselves in; one is in custody, and the rest are being sought, Parrott said.