Minneapolis bridge inspectors John Beetsch and Kent Madsen spent a half-day in October on the Martin Olav Sabo Bridge, examining the 18 sets of cables and cable anchors, then giving them the highest rating for soundness and lack of corrosion.

"Cables are good," their report on the four-year-old bridge noted.

So Beetsch said on Tuesday he was shocked after learning Sunday evening that the longest and tallest set of cables supporting the span's east side had broken loose from the mast, prompting the steel cable anchor beneath it to fracture.

He said he felt relieved that no one was hurt, but after rushing to the scene, "You just start wondering how this could happen."

Officials say it is still too early to identify a cause for the failures on the city-owned pedestrian and bicycle bridge. The emergency closing of the distinctive cable-stayed bridge has diverted vehicles off Hiawatha Avenue and disrupted service of the Hiawatha light-rail line indefinitely, as workers trying to shore it up encounter unexpected obstacles.

This much is known: Minneapolis officials gave the cable system on the 2,200-foot span -- and all of its other parts -- top marks in every inspection going back to 2008.

An additional inspection of the cables in October, aimed at going beyond the annual inspection, also gave them a thumbs-up.

That's as it should be, said Beetsch, noting that it is unlikely for a bridge opened as recently as late 2007 to have problems. He added that inspectors used a lift to take them 100 feet from the bridge to the top of the mast to study the same anchors that are now compromised.

Engineers will look into whether the bridge's design, construction or some outside force caused the cable supports to crack. Mike Kennedy, director of transportation, maintenance and repair for the Minneapolis Department of Public Works, said such an investigation typically takes months.

The city is getting assistance from the bridge's design consultant, San Francisco-based URS Corp. The engineering firm was also a consultant on the state-owned Interstate 35W Bridge that collapsed in 2007, killing 13 people and injuring 145. 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.

City workers have been stymied in their attempts to install timber and steel shoring structures on the east end of the Sabo bridge because the soil is too soft. They can't fix the compromised anchor without ensuring the span is sound, according to Kennedy. He said concerns remain about the second cracked cable support, which is still connecting a pair of cables to the mast.

"We are having to sort of rethink the plan ... and it's all taking time," Kennedy said.

The breakage of the top set of cables on the east end of the mast is significant because the cables on that end support a greater part of the bridge than those on the other side, which are anchored into the ground with concrete blocks, according to city officials.

Efforts to stabilize the bridge also involve the Minnesota Department of Transportation and Hennepin County, which oversaw construction of the $5.1 million structure before Minneapolis acquired it in 2008.

As officials work to avoid further damage, commuters in that area will face delays. Metro Transit is operating replacement buses at the 38th Street, Lake Street/Midtown and Franklin Avenue stations on the Hiawatha light-rail line that runs near the bridge "until further notice."

Maya Rao • 612-673-4210