Harris said no refunds were made due to the audit because officials regard it as "inconclusive." "We fundamentally don't agree that the URS audit is a complete, comprehensive audit," Harris said. "It's irresponsible in our opinion to issue refunds because it's inconclusive."

Instead, Harris said, the administration better trusted a Xerox review of about 7,000 of its tickets. The city has said that study — also never released — was conducted in December 2012, after the city had decided to use a different vendor amid a Baltimore Sun investigation of errors in the program. Xerox said at the time that it had detected a 5.2 percent error rate at five cameras, and the city issued about 350 refunds.

The URS audit identified several cameras with very high error rates. A camera in the 1000 block of Caton Ave. had a 35 percent error rate, the audit found. A device at the 6500 block of Eastern Ave. had a 45 percent error rate. And a speed camera in the 5400 block of Loch Raven Blvd. had a 58 percent error rate.

The audit was done as part of $278,000 of work URS conducted for the city last year. Among other work, the firm also monitored testing aimed at fixing and restarting the camera system under a second speed camera company, Brekford Corp.

Its findings — which the city made public in response to a public records request from The Sun — showed persistent problems in the Brekford system, including preventable errors.

City Solicitor George Nilson has said releasing the URS audit of Xerox would violate a settlement agreement with that company.

This month, the city expanded its contract with URS. The Board of Estimates agreed to pay $237,000 for "additional independent monitoring services" of the city's speed and red-light cameras. The company will monitor "engineering services, documents and preparing of standard operating procedures and business rules," according to board records.

Baltimore Sun reporter Yvonne Wenger contributed to this article.