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Senior national security officials said they have been briefed about a threat that a plane would crash into the Capitol in Washington D.C. on Wednesday, but told Fox News that the threat did not seem to be "credible." 

CBS News first reported the threat.

"We are flying a plane into the Capitol on Wednesday," an audio recording of the threat sent to air traffic controllers in New York, obtained by CBS News, said. "Soleimani will be avenged."

The threat referred to Qassem Soleimani—the Iranian general killed in Iraq last year in a U.S. airstrike ordered by President Trump. American officials deemed Soleimani — the head of Iran's Islamic Revolutionary Guard Corps' elite Quds Force — as dangerous as the Islamic State and held him responsible for the deaths of hundreds of U.S. troops. 

A national security official told Fox News that "relevant agencies" were briefed on Tuesday's incident and that "while it is possible it is a hoax, it is being taken seriously."

And a senior Defense official told Fox News that they take "all threats seriously," but that "this does not seem to be credible, though we'll continue to assess." 

Another senior U.S. official told Fox News that the threat "doesn't seem legit." 

"We're not overly concerned," the official said. 

In a statement to Fox News, the FBI said it "takes all threats of violence to public safety seriously." The Capitol Police was working with federal law enforcement partners. 

The Federal Aviation Administration works with authorities and national security partners on "reported security threats that may impact aviation safety," it said in a statement.

The threat comes as Congress is expected to meet Wednesday in a joint-session to certify President-elect Joe Biden's win in the November presidential election. 

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Iran has vowed revenge for the Jan. 3, 2020 killing. At the time, Tehran retaliated with an airstrike of its own on a military base in Iraq, resulting in concussions for up to 100 U.S. troops. 

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In a Tuesday tweet, Iran's Foreign Minister Javad Zarif remembered Soleimani, saying he was "cowardly assassinated by the terrorist-in-chief."

Fox News' Jennifer Griffin contributed to this report.