In February, then-new Suffolk County Police Commissioner Tim Sini promoted Richard Mamay to lieutenant, even though Mamay was under internal investigation.

Mamay was accused of allowing the stepson of a colleague a third try at an agility test recruits must pass. Two tries was the limit. The recruit should have been sent packing.

Sini said then he was willing to promote Mamay because he knew it could be reversed as long as Mamay was in his probationary period.

Now, Sini has learned a lesson about how disciplining works, the power of police unions and the state law 50a that cloaks all police personnel matters in secrecy.

The investigation concluded Mamay had broken the rules and Sini tried to demote him. But then Mamay and the Suffolk County Police Benevolent Association sued to stop the demotion and Sini backed down, settling the matter. Mamay will keep his title, but has agreed to give up 60 days of accrued leave time worth more than $40,000.

Mamay’s offense is a window into the way the department shepherds favored friends and relatives onto the force.

It’s been a particular problem in Suffolk, where the police force does not adequately mirror the minorities in the county. Sini does say he’s combating that. The department is under a federal order to take 10 percent of new recruits from a list of Spanish speakers, he says civil service workers will now be present as recruits are tested, and he says national recruiting that includes black college students and military members will help.

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Unfortunately, the new commissioner cannot even disclose what happened in the Mamay case because the State Legislature has not reformed the 50a rule to give the public more information about misconduct.

But at least the Mamay case has freed Sini of a pesky naiveté about the power of police unions to tie his hands.