TULSA, Oklahoma - An investigation by News On 6 and our partner, The Frontier, found Tulsa Police officers "buying stripes," or paying for a promotion.

Sources tell News On 6 it's a practice that's been happening for decades – officers buying the rank of corporal, sergeant, captain, major and deputy chief for anywhere from $20,000 to $50,000. One former officer said it dates back to the 60s.

Here's how it works:

A supervisor lets it be known he or she might retire, for a price, then shops around for an officer who wants to move up in rank; those officers have already tested to qualify for the promotion.

The officer with the highest test score is first on the list to promote when someone retires. But officers are required to re-test each year, so an officer could top the list for promotion one year and drop the next, risking the chance to promote.

FOP President Clay Ballenger said, "There have been promotion lists where, I understand, they are very close, let's say, between the top five."

Police Chief Chuck Jordan knows some officers are buying their promotions - promotions that could earn them $16,000 more a year, and, eventually, a higher pension.

One 35-year officer said there's nothing "morally wrong" with it, saying, "Who's the victim? Two people get together and make a financial arrangement." He went on to say, "I don't know how you'd prevent it. You'd have to tear up our whole system."

A major told us, "I might consider doing it if I knew it were legal, but I wouldn't pay thousands of dollars."

Ballenger said, "You know, I've been told that it's not illegal and that it's been vetted through city legal."

Jordan said he asked the city attorney in 2010 for a legal opinion about the practice and was told it’s legal and does not violate policy. A former city attorney, and the current city attorney, said there is no documentation of that.

Republican for Tulsa County sheriff, Vic Regalado, said he paid a supervisor to retire so he could be promoted to sergeant. Democrat for Tulsa County sheriff, Rex Berry, said he knew about the practice when he was an officer in the 70s, but did not participate.

The city's ethics ordinance states:

"It is the policy of the City of Tulsa that the proper operation of democratic government requires that public officials and employees be independently impartial and responsible to the people of the City; that government decisions and policies be made only through proper channels of the governmental structure; that no City official should have any interest, financial, personal, or organizational, direct or indirect, or engage in any business, transaction, or activity or incur any obligation that is in conflict with the proper discharge of their duties in the public interest; that public office and public employment are positions of public trust imposing the duty of a fiduciary upon all officers, employees, members of boards and committees, and trustees of public trusts with the City of Tulsa as a beneficiary; and such individuals shall not use their public positions for personal gain nor should they act in such a way as to give an appearance of any impropriety."

Our partner, The Frontier, has more on this story, and we will continue to follow this in the coming days.