Rick Gates is the latest to fall before special counsel Robert Mueller’s investigation into the Trump presidential campaign’s Russia ties.

He pleaded guilty today in a Washington, D.C. courtroom (Feb. 23) to charges of conspiring against the United Sates and lying to investigators.

Gates was the number two to former campaign chair Paul Manafort, and the second most senior person in the Trump campaign to plead guilty, following former national security advisor Mike Flynn. Gates’ plea will make it five different people to have pleaded guilty to charges brought by Mueller.

Last year, Gates pleaded not guilty alongside Manafort, his longtime business partner, to 12 charges. Mueller upped the stakes this week by filing another 32 charges (paywall) against the two men, alleging tax fraud and other illegal financial tricks—this time to be tried in a separate case in Virginia.

In a letter to his family obtained by ABC News, Gates described his “change of heart” to plead guilty, apparently motivated to protect his family from further public exposure and a costly legal process.

As part the plea deal, Gates agreed to cooperate with Mueller’s probe, increasing the pressure on Manafort, his former mentor who issued a statement insisting he is not guilty. He is reportedly expecting to go to trial over financial crimes allegedly committed while working in the Ukraine, before the Trump campaign started.

Manafort was fired from the Trump campaign after those financial ties were reported in summer 2016, but Gates worked for the Republican National Committee until the election and then took a senior position on Trump’s inaugural committee. He then went to work for a close friend of Trump’s and reportedly retained access to the White House until at least June of last year.

If he witnessed any criminal activity during the campaign or knows anything incriminating about campaign members, Mueller’s team likely knows it. CNN reports that Gates has had a “Queen for a Day” interview, in which a defendant agrees to answer any questions prosecutors have on their case. It’s likely that interview played a part in the indictment of Alex Van Der Zwaan, a Dutch lawyer who pled guilty to lying to federal prosecutors about his interactions with Gates and another unnamed individual.

Those who have pleaded guilty to Mueller charges include former Trump campaign aide George Papadopoulos and Richard Pinedo, who sold US citizens’ bank account numbers to Russian internet trolls.

Racking up yet one more guilty plea won’t help the Republican campaign to discredit Mueller’s investigation as baseless and politically motivated. Trump reacted to the Gates-Manafort indictment in October by complaining that the events took place “years ago,” but the indictment actually says the alleged crimes were committed between “at least 2006 and 2015.”

Mueller’s charges against Gates and Manafort go back to their time working as spin doctors for former Ukrainian president Viktor Yanukovych, who later fled to Russia. Their alleged crimes include money laundering, failing to register as lobbyists of a foreign power under the Foreign Agents Registration Act, and lying to FBI and Department of Justice officials.