An independent arbitrator says the city of Chicago owes some 3,100 city police officers $1 million in overtime pay for their work during the NATO summit last year.

Arbitrator Steven M. Biering ruled that the city's contract with police officers “unambiguously provides that an officer who works six or seven consecutive days in a work week shall be paid overtime for work performed on the sixth and seventh days.”

“The city violated the contract by failing to pay overtime to officers who worked a sixth or seventh day during the week of May 20,” of 2012, Bierig continued. 

The mayor argued that he did not have to pay the overtime, saying that paying an officer overtime for working a day of work that was originally scheduled off and then paying overtime for a sixth and seventh consecutive day of work is the equivalent of "pyramiding or payment of double overtime."

But in his ruling, Biering said the city's contract with the Fraternal Order of Police “does not provide an exception for payment of sixth and seventh day overtime in weeks when officers are paid overtime for cancelled regular days off.”