A 69-year-old former Oklahoma police officer was sentenced Tuesday to two years in prison for training clients to successfully cheat on lie detector tests.

Douglas Williams pleaded guilty in May to five counts (PDF) of obstruction of justice and mail fraud in connection to his now-defunct polygraph.com website and consulting business. According to an indictment, Williams instructed customers how to beat lie detector tests given during national security, federal, state, and local employment suitability assessments and for internal federal investigations.

On a YouTube video, Williams tells potential customers that "I can teach you how to pass, nervous or not, no matter what." He also says, "Remember, failing to prepare is simply preparing to fail."

According to court documents, the government said he told the following to an undercover agent: "I've taught a lot of those guys. In fact, there's a lot of government agents—FBI, Secret Service, NSA, all of those alphabet agencies—that have already retired, that I taught, years ago. And I know what I'm doing, and you will pass with no problem."

This isn't the first polygraph-cheating prosecution we've seen. An Indiana man was accused of offering similar services and was sentenced in 2013 to eight months in prison.

Polygraphs were invented in 1921. Their results generally are not admissible in court. Discussions about how to beat them are online.