CHICAGO (AP) _ A former city water official pleaded guilty Friday to racketeering and tax fraud, becoming the highest-ranking city official brought down by the government's 18-month corruption investigation at City Hall.

In a plea agreement, Donald Tomczak would be sent to prison for nearly four years and agreed to cooperate with the federal investigation into Chicago's scandal-plagued $38 million truck-for-hire program, which has the city hiring out hauling work to private truckers.

Tomczak, the former first deputy water commissioner, was accused of furnishing jobs and preferential treatment to trucking companies in return for payoffs, gift certificates for himself and others and campaign contributions to an unnamed relative and a candidate for the city council.

For years, Tomczak was a Democratic precinct captain in the 11th Ward _ where Mayor Richard M. Daley's brother, John, is head of the Democratic Party.

Tomczak was the first to admit publicly any involvement in what officials say has been a massive violation of a decree banning the use of political clout in hiring for city jobs.

More than 20 people have pleaded guilty in the investigation, including trucking company owners and city officials who admitted that payoffs were exchanged in return for work in the program.

Mayor Daley has said the Hired Truck Program has been reformed since the scandal broke. U.S. Attorney Gary Shapiro said there was nothing in Tomczak's 31-page plea agreement to suggest that the mayor had done anything wrong.

``On the other hand, we never say people do or don't,'' Shapiro told reporters.