A U.S. defense official confirmed to ABC News Tuesday that President Joe Biden will withdraw all U.S. forces from Afghanistan by Sept. 11, the 20th anniversary of American involvement in the country’s longest war.

There are roughly 2,500 U.S. troops in Afghanistan now. American troop levels reached a high of 100,000 troops in August 2010 and stayed at that level for much of the next year.

Biden will announce the withdrawal date on Wednesday, a senior administration official said Tuesday.

The senior administration official told reporters on a conference call that the drawdown would begin before the end of this month and could finish before the 20th anniversary of the September 11th attacks, which the official called “the outside date by which it will be completed.”

The official said that the number of troops would be reduced to zero, and that the withdrawal would not be based on conditions on the ground.

“This is not conditions-based,” the official said. “The president has judged that a conditions-based approach, which has been the approach of the past two decades, is a recipe for staying in Afghanistan forever.”

U.S. intelligence agencies have determined that Al Qaeda does “not currently possess an external-plotting capability that can threaten the homeland,” the official said. Repositioning troops would help the U.S. “focus” on “a dispersed and distributed terrorist threat,” according to the official.

“This is not 2001,” the official said. “It is 2021, and in 2021, the terrorist threat that we face is real and it emanates from a number of countries, indeed a number of continents, from Yemen, from Syria, from Somalia, from other parts of Africa.”

The development was first reported by The Washington Post.

Last month, Biden said it would be “tough” for the U.S. to meet a May 1 deadline to withdraw troops from Afghanistan.

The May 1 deadline was part of an agreement reached by former President Donald Trump and the Taliban.

More than 2,200 U.S. troops have died and another 20,000 have been wounded since October 2001.

This is a developing story. Please check back for updates.