Miami-Dade County engineers knew that, sooner rather than later, they would have to replace the 69-year-old portion of the bridge to Key Biscayne.

They just didn’t know how soon. Facing a $25 million bill to rebuild the structure, they didn’t make the project a priority until the state notified them last year that the Bear Cut Bridge had deteriorated so badly over the previous two years that the problems could become critical.

Now, a vital artery to Key Biscayne has been squeezed to makeshift lanes for likely a year or more.

As Miami-Dade fast-tracks a plan to repair the bridge, county commissioners who say the bridge should never have been allowed to reach such a state of disrepair are wondering who screwed up: the Florida Department of Transportation, which inspects the bridge, the Miami-Dade public works department, which maintains it, or both.

“It doesn’t happen overnight, this deterioration,” Commissioner Bruno Barreiro said at a regional transportation committee meeting last week. “Either FDOT or somebody dropped the ball there.”

County and FDOT engineers have defended their work. The state agency says it followed federal protocols requiring bridge inspections every two years. The county says it made repairs as needed.

“The inspections on the bridge were ... performed in 2006, 2007 and 2010,” Kathleen Woods-Richardson, the public works department director, told Barreiro last Monday. “Those did not disclose anything. The one in 2012 was the one that discovered the deficiencies.”

Gus Pego, FDOT’s Miami representative, said the bridge has been inspected “many, many, many times” since it was built in 1944.

“So then why do we have an emergency?” Commissioner Juan C. Zapata asked.

No one was able to give a reason, other than sometime between 2010 and 2012, corrosion on the bridge’s exposed steel beams significantly deteriorated. Despite the bridge’s age and expected lifespan of 75 years, the state was not inspecting the bridge once or twice a year, as some bridges are when they are deemed deficient.

“There was no indication to indicate to us that there was a need to put it on a one-year cycle,” Pego said. “Usually there is a slower deterioration.”

FDOT first rated Bear Cut Bridge “structurally deficient” in 2008 — a designation that signaled the bridge would require serious repairs or replacement within six years.

Ten other bridges have that rating currently in Miami-Dade, including the Miami River drawbridges at Southwest First Street and at Miami Avenue, and the Broad Causeway at Northeast 123rd Street. Thirteen bridges have the same rating in Broward, including the Sunrise Boulevard bridge over the Middle River in Fort Lauderdale.

The 2008 rating on the Bear Cut Bridge was a result of problems with the bridge’s pilings that hold the structure up in the water, and not with the steel beams, according to the county. Miami-Dade made nearly $600,000 worth of piling repairs from 2008 to 2009.

The bridge was no longer rated deficient in its next state inspection, in 2010. Still, that January, the county put rebuilding the bridge and the nearby West Bridge, located immediately after the Rickenbacker Causeway, on a list of upcoming needs. No money was set aside for the projects.