Five years ago Monday, 11 men died on the Deepwater Horizon oil rig—despite Chelsea Manning’s effort to save their lives.

Let me explain.

The BP drilling rig blew itself to kingdom come after the “mud”—the cement used to cap the well—blew out.

The oil company, the federal government and the industry were shocked—shocked!—at this supposedly unexpected explosion in the deep waters of the Gulf of Mexico.

But BP knew, and Exxon and Chevron knew, and the U.S. State Department knew, that just 19 months earlier another BP offshore rig suffered an identical, disastrous blowout halfway across the planet in the Caspian Sea.

In both the Gulf and Caspian blowouts, the immediate culprit was the failure of the cement, in both cases caused by the use—misuse—of nitrogen in the cement mix, a money-saving but ultimately deadly measure intended to speed the cement’s drying.

The cover-up meant that U.S. regulators, the U.S. Congress and the public had no inkling that the cost-saving “quick-dry” cement process had failed on an offshore rig until the Deepwater Horizon blew.

But Pvt. Chelsea (formerly Bradley) Manning tried to warn us. The details of the Caspian Sea blowout off the coast of Baku, Azerbaijan, were revealed in the secret State Department cables Manning released in December 2009 through WikiLeaks. Cables from the U.S. ambassador relayed a summary of confidential meetings in which BP’s top Azeri executive confided that the big Caspian offshore rig suffered a “blowout” in September 2008, leading to the “largest such emergency evacuation in BP’s history.” Its likely cause: “a bad cement job.”

The message was relayed to Washington after BP’s American partners in the Caspian—Exxon and Chevron—asked the State Department to find out why BP had ceased to drill in the Caspian, costing them all millions. State, then headed by former Chevron board member Condoleezza Rice, got the oil chiefs their answer—then joined them in keeping it secret.

(Not knowing about the Manning cables, I had to find out about the Caspian blowout the hard way. Just days after the Deepwater Horizon blowout, I received a tip from an eyewitness to the Caspian disaster. To determine the facts I flew to Baku, where my British TV crew and I found ourselves placed under arrest by a team of goons from the Azerbaijan secret police, the military and some of BP’s oil-well-insignia-sporting private security clowns. As a reporter for British television, I was quickly released—with the film of the bust captured on my little pen camera. But, terribly, two of my rig-worker witnesses disappeared.)

Had BP or the State Department ’fessed up to the prior blowout—a disclosure required by U.S. and British regulations—it is exceptionally unlikely that BP would have been allowed to use the quick-dry cement method in the deep Gulf of Mexico.

Indeed, there may have been a complete prohibition on the drilling, because Department of Interior experts had opposed deep drilling in that part of the Gulf. To lobby the government to allow drilling there, about six months before the Deepwater Horizon blew, BP executive David Rainey and the presidents of Exxon USA and Chevron testified before Congress that offshore drilling had been conducted for 50 years “in a manner both safe and protective of the environment.”