Matt Coyne and Thomas C. Zambito | The Journal News

Peter Carr and John Meore, Rockland/Westchester Journal News

A piece of the old Tappan Zee Bridge became destabilized Friday, creating “a potentially dangerous situation," which caused state officials to call off plans to open the Westchester-bound span of its replacement, the Gov. Mario M. Cuomo Bridge.

Matthew Driscoll, the executive director of the New York State Thruway Authority, said engineers disassembling the Tappan Zee feared the destabilized piece of the old bridge could fall.

“Given its proximity to the new completed span, out of an abundance of caution, motorists will remain in the current traffic configuration until a thorough evaluation by Tappan Zee Constructors is complete,” Driscoll said Saturday morning.

It’s unclear when the new eastbound span will open.

“The second span is finished and ready to open to traffic as soon as the Thruway Authority is assured there is no risk to the new span,” he added.

The discovery prompted the Coast Guard to close the navigational channel under the bridge and the traffic shift already underway was canceled late Friday.

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One channel was opened to water traffic around noon on Friday, but vessels can only travel under the bridge one at a time and only in daylight hours.

"I believe some engineers from the construction company were very concerned about it," Rockland County Sheriff Louis Falco told The Journal News/lohud.com about the Tappan Zee Bridge remnant.

Since all traffic was moved onto the $3.9 billion Gov. Mario M. Cuomo Bridge opened last October, workers have been busy dismantling its predecessor.

Much of the above-water work has been done, but large pieces still remain, including chunks of the main span.

Mark Vergari/The Journal News

Tappan Zee Constructors -- the consortium of engineering firms enlisted to design and build the new twin span bridge and tear down the 62-year-old Tappan Zee Bridge -- did not return repeated requests for comment overnight. Falco said the company wanted to take a better look at the structure in the daylight.

State officials did not provide details on the piece of the old bridge that became stabilized.

For much of the late evening and early morning, law enforcement boats were on the Hudson River, just north of the bridge.

After a ceremony headlined by former Democratic presidential nominee Hillary Clinton and featuring Franklin Delano Roosevelt's 1923 Packard convertible, Westchester-bound traffic was set to transition to the Gov. Mario M. Cuomo Bridge's second span.

Shortly before 9 p.m., crews closed two lanes headed toward Westchester on the run up to and over the bridge.

That pattern was in place until around midnight, when the cones were picked up and traffic shift operations were called off.

News the second span would open came as a surprise to many, with Gov. Andrew Cuomo taking an impromptu tour of the bridge Aug. 27, where he announced the bridge would open in "mid-September."

Tuesday, on a boat tour and press event, Cuomo said the bridge would open Friday night into Saturday morning, weather permitting.

The timing of the opening, less than a week before his primary with Cynthia Nixon for the Democratic gubernatorial nomination, had some questioning if it was done to bolster his campaign.

"The bridge is ready to open," Cuomo said when asked about the political implications Tuesday. "Unless you think we should keep (people stuck in traffic)."

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Governor Mario M. Cuomo bridge second span ceremony