Moody’s Investor Services downgraded Evanston’s credit rating from the highest possible score Thursday, citing the city’s pension liabilities.

The lowering from Aaa to Aa1 by the bond credit rating agency was caused in part by more conservative calculations of the pension liabilities of Evanston firefighters and police, said Marty Lyons, the city’s chief financial officer. The projections reduced the amount of revenue in the employees’ trust funds, which led to increased unfunded liabilities.

The city has taken measures to ease pension liabilities by using more conservative actuarial measurements to more accurately reflect future assets and liabilities of the funds.

Among the city’s economic strengths, Moody’s pointed to its “affluent and diverse tax base” courtesy of local health care institutions and Northwestern. The agency also said the city is led by officials who can be trusted to tackle the pension issue.

There are currently 17 communities rated Aaa and 22 rated Aa1 in Illinois, according to a news release.

— Edward Cox