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Image Ludwig Paz

For years, a curious pattern played out across Brooklyn, Queens and Long Island.

Neighbors would complain about a brothel in their building or on their street. The police — some undercover, some with battering rams — would show up, but would leave empty-handed.

Until recently, nobody could definitely say why.

Then Ludwig Paz walked into a courtroom this week in Queens and pleaded guilty to promoting prostitution and other crimes.

The authorities said Mr. Paz learned what he needed to know about running his brothels and his criminal enterprise from an earlier job: detective with the New York Police Department.

The brothels have been described as one of the worst police scandals in recent years

In September, the police arrested 50 people in connection to the brothels. They included Mr. Paz, three sergeants, two detectives and two officers with the Police Department.

Two additional police officers were stripped of their guns and badges and placed on administrative duty.