MAKE NO mistake about it: The corruption investigation that has engulfed Hialeah's police department and forced the early retirement of Chief Cecil Seay and the suspension of Sgt. Leo Thalassites is extremely serious. If the allegations against these men are true, this is not a case of one or two police officers who strayed from the straight line.

Wiretap and other evidence indicates that some Hialeah police officials were under the pernicious influence of convicted murder conspirator Alberto San Pedro, 36. Metro-Dade investigators term him a "major corruptor in Hialeah." That prospect is considerably more serious than one of internal police corruption -- itself serious enough.

His critics blame much of the police department's difficulty on Hialeah Mayor Raul Martinez, who is not implicated in the widening scandal. These critics accuse the mayor of stacking the selection process two years ago in order to appoint his preferred candidate, Mr. Seay, as chief. They also fault Mayor Martinez for not intervening sooner to try to stem the damage as evidence mounted of San Pedro's corruptive influence inside the police department.

When his house is on fire, the prudent person doesn't stand around debating the cause. He sounds the alarm. It's pointless to debate what Mayor Martinez did or didn't do two years ago. The priority now is to put out the fire in Hialeah's station house.

Mayor Martinez has named himself interim police chief and assumed control of the police department. That's commendable -- but around him are embers that won't be easy to extinguish. He may -- and he certainly should -- come to regret telling Hialeah police officers who complained about low department moral to resign if they don't like it. A department under siege cannot endure an internecine battle between the mayor and police officials without irreparable damage.

The mayor should do three things. First, he should stop trying to portray the criticism of Hialeah and its police department malaise as unfair. Hialeah's problems are serious, and they can only be overcome if they are acknowledged as such. Second, the mayor-cum-police chief needs to mend fences with his police officers.

His best mortar for that job lies in his third imperative: He should ensure that the selection of the new police chief is above reproach. Only a first-rate police official, probably one from outside the department, can solve the long-range problems of Hialeah's police department.