The United States destroyed all of its stockpiles of chemical weapons.

The Pentagon announced this on its website.

“The U.S.’s destruction of declared chemical weapons stockpiles in a safe, secure and environmentally responsible manner was a difficult task, requiring many years to complete,” said Douglas Bush, Assistant Secretary of the army for acquisition, logistics, and technology.

Nearly 90 percent of the weapons were eliminated by 2012. The last 10 percent was a greater challenge involving a complicated approach of neutralizing these chemicals.

The last mustard gas munition was destroyed last month at the Army’s Pueblo Chemical Depot.

Blue Grass destroyed the last missile loaded with Sarin nerve agent last week.

The cost of the entire project to destroy stockpiles of chemical weapons reaches $13.5 billion.

“Through last Friday, the United States destroyed over 30,000 metric tons of declared chemical agent contained in nearly 3.5 million chemical munitions, over 22,500 one-ton containers containing chemical agent, and over 50,500 bottles and containers containing chemical agent,” said Kingston Reif, Deputy Assistant Secretary of defense for threat reduction and arms control.

The destruction cements U.S. compliance with the Chemical Weapons Convention by the September 30 deadline.

The Senate ratified the Chemical Weapons Convention in 1997, and DOD employees started the process of safely demilitarizing the weapons.

Two years of efforts remain to ensure that the facilities used are safe before turning out the lights. Officials said that will cost another $2.5 billion.

The U.S. Department of Defense cooperates with other countries in this direction, but expresses doubts about Russia and Syria implementing the convention, as they have been repeatedly using chemical weapons.