Minneapolis engineers found no sign of trouble during their last inspection of the 18 sets of cables that support the Martin Olav Sabo Bridge and make it a striking landmark over Hiawatha Avenue.

Five months later, a cable broke loose from the top of the span and fell about 100 feet onto the bridge.

No one was hurt when the cable fell Sunday night, but safety concerns prompted the bridge's closure, re-routing traffic, interrupting Hiawatha light-rail service and sending more than 20 city workers on overtime to shore up the bridge.

Those disruptions continued into Tuesday morning.

The $5.1 million, 2,200-foot bridge opened in November 2007 as Minnesota's first cable-stayed bridge, offering bicyclists and pedestrians an alternative to dodging traffic on Hiawatha Avenue. The most recent inspection, in September, turned up "nothing we saw that got anyone's attention, so we are just as surprised as anybody," said Mike Kennedy, director of transportation, maintenance and repair for Minneapolis Public Works.

A pedestrian came across a cable on Sunday night and reported it to the city, said Kennedy. The set of cables that failed are the bridge's longest and reach to the very top of the tower.

Kennedy predicted it will be "some time" before the bridge reopens. He said it was too early to speculate about the cause.

City workers erected several timber and steel columns, known as shoring structures, to support part of the span near the rail line on Monday, but Kennedy said work was moving "very, very slowly."

The bridge's design consultant was San Francisco-based URS Corp., an engineering firm that consulted on the Interstate 35W bridge that collapsed in 2007. URS agreed in 2010 to pay $52.4 million to settle the last major piece of litigation brought by victims, who accused the company of missing warning signs in the bridge before its rush-hour collapse into the Mississippi River. Thirteen people died and 145 were injured.

URS was hired by Hennepin County to design the Sabo bridge before the collapse.

A spokesman for URS said in an e-mail that the company is working with local officials "to evaluate the safety and stability of the bridge so that light rail service through the area may resume and Hiawatha Avenue be reopened."

Burnsville-based Ames Construction built the Hiawatha span, but an executive there said that subcontractors did the cable work.

"We weren't aware of any problems with the project, and we're surprised to hear the news," said Roger McBride, the company's vice president of safety, risk management and human resources.

Ames' website says the Sabo bridge is unique because the weight of the main span is supported by cables anchored into concrete blocks attached to limestone bedrock 90 feet below ground.

Given that the light-rail line runs next to that side of the span, transit officials also suspended service at the Lake Street station. Northbound commuters had to get off the trains at the 38th Street station on Monday, transfer to a bus serving the Lake Street and Franklin Avenue stops, and get back on the train at Franklin Avenue.

On Hiawatha Avenue, vehicle traffic will continue to be off limits from Lake Street to 26th Street.

The bridge was paid for by a federal grant championed by Martin Olav Sabo, a longtime U.S. House member from Minneapolis who retired in 2007. Additional funds came from Hennepin County, which gave the bridge to the city after it was built. It was named for Sabo in May 2008.

Before it was constructed, an increasing number of bicyclists on the Midtown Greenway had to make a sometimes dangerous crossing over Hiawatha, a six-lane thorough-fare.

The Sabo bridge "radically increased safety," said Soren Jensen, executive director of Midtown Greenway Coalition.

Maya Rao • 612-673-4210 Paul Walsh • 612-673-4482