For 21 months, I’ve suspected that District Mayor Vince Gray was lying when he said he knew nothing at the time about the extensive criminal wrongdoing during his 2010 campaign.

Now I’m sure of it.

Businessman Jeff Thompson’s sworn accounts Monday in federal court provide the last piece of evidence needed to convince me that Gray has been deliberately deceiving the public and continues to do so.

In particular, Thompson described two face-to-face meetings with Gray in June and August of 2010 in which they discussed secret funding for Gray’s campaign.

That was crucial testimony because it ended Gray supporters’ hope that Thompson wouldn’t say Gray had personal knowledge of the scheme.

Gray’s position now is that Thompson made it all up. He says the businessman lied to get a light sentence, expected to be six months or less in prison, when he pleaded guilty to two felony counts.

“Who do you believe?” Gray said Tuesday evening in his State of the District address. “A greedy man attempting to save himself, or me, a public servant who has dedicated his entire career to giving back to our communities?”

Sorry, Mr. Mayor. I greatly respect your career of service. I wrote a column in December 2009 saying I hoped you’d run for mayor. You phoned to thank me.

But I believe Thompson, for multiple reasons.

First, Thompson isn’t alone in making the allegations. Another dedicated public servant, U.S. Attorney Ron Machen, is driving this effort.

The prosecutor’s credibility is very much at stake. Machen signed the 33-page statement of offense, which included Thompson’s account of Gray’s alleged direct role in the conspiracy to evade campaign finance laws.

If I have to choose between trusting Gray or Machen, I go with the latter. He has an impressive record in this scandal. Five people involved in Gray’s campaign, plus three former D.C. Council members, have pleaded guilty to felonies and agreed to cooperate with the government.

Gray’s defenders like to whisper that Machen is a rogue prosecutor with a vendetta against the mayor. But former U.S. prosecutors praised his work as highly professional.

“It’s meticulous, and it’s methodical,” said Roscoe C. Howard Jr., a partner at Andrews Kurth, who was U.S. attorney for the District from 2001 to 2004. “He is going through this the way an investigation should go through.”

Moreover, Machen has other witnesses to help validate Thompson’s account of what happened. Federal prosecutors typically do not rely on a single witness, especially one whose credibility is easy to challenge, so it’s a good bet there’s other evidence, too.

For instance, we know already from the statement of offense that at least two cooperating witnesses besides Thompson have some knowledge of the August dinner where Thompson said the mayor personally asked for more than $400,000 for the “shadow campaign.”

“You want to make sure that the people who are taking the plea don’t later make you look foolish by giving you a story that you can’t back up,” Howard said.

If Gray ever offers his own detailed account of what happened, he might argue that he believed the support from Thompson was legal.

But that’s not remotely credible. The mayor knew the law. He knew one can’t receive more than $660,000 in support that doesn’t get reported.

It’s undeniable that Thompson is getting off easy, but that’s how the legal system works. Prosecutors must be able to offer lesser punishments to those willing to testify to nab corrupt officials.

“In order to get the mayor, they had to get a deal with Jeffrey Thompson, and they got a good one,” said Joe diGenova of diGenova & Toensing, another former U.S. attorney in the District.

Even if Gray is lying, it doesn’t mean a jury would necessarily convict him if he were indicted. He’s innocent until proven guilty, and the evidence has to be beyond a reasonable doubt.

On the other hand, based on the plea agreement and Machen’s comments at a news conference afterward, I predict the prosecutor will eventually bring charges against the mayor.

I suggested to Machen at the news conference that with Thompson’s plea, he already has enough evidence to indict the mayor on conspiracy.

“We corroborate every aspect of our investigation,” Machen said. “We’re still running down some loose ends.”

Meticulous to the finish.

I discuss local issues Friday at 8:50 a.m. on WAMU (88.5 FM). For previous columns, go to washingtonpost.