An independent New York City panel was formed to investigate and discourage police brutality and other misconduct.
NYPD spokesman Paul Browne dismissed the criticism. He said the group "distorted or ignored facts to reach its predetermined conclusions."
The police department has been sharply criticized over the years for various violent incidents, including the jailhouse torture of Abner Louima in 1997 and the 1999 killing of unarmed African immigrant Amadou Diallo, who was struck by 19 of the 41 bullets fired at him.
According to the ACLU report, the review board saw a 60 percent increase in complaints between 2000 and 2005, from 4,251 to 6,796. The report said the board investigated fewer than half of the allegations and typically found the complaints valid only about 5 percent of the time.
Of the most serious cases referred to the NYPD for disciplinary action between 2000 and 2004, Police Commissioner Raymond Kelly rejected the board's recommendations in 63 percent of the cases. "When discipline was imposed, it was strikingly lenient in light of the severity of the misconduct that has been documented by the CCRB," the report said.
Police officials attribute the spike in reports of misconduct to the introduction of the city's complaint telephone hot line. They said the department has seen a decline in excessive force complaints - and thus a corresponding decrease in suspensions or firings.
The NYCLU recommended more stringent City Council oversight of the board along with increased funding for investigators and support staff.
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