OAK RIDGE, Tenn. (AP) — Demolition workers have taken down the last building in a massive Oak Ridge uranium enrichment complex built more than 70 years ago as part of the top-secret Manhattan Project.

The Knoxville News Sentinel reports (http://bit.ly/2bXtn45) Sen. Lamar Alexander was among those commemorating the milestone on Tuesday. He says the demolition paves the way for new economic development at the site now called the East Tennessee Technology Park.

The cleanup of five gaseous diffusion buildings used to enrich uranium at the site cost $1.7 billion.

The remaining cleanup at East Tennessee Technology Park is slated to be complete in 2020.