Now that they’ve made their impeachment investigation official, the House Judiciary Committee is calling its first witness: Corey Lewandowski. The main line of questioning will likely be about the extent of his role in President Donald Trump’s efforts to obstruct justice.

But there should also be questions about the Trump campaign’s collusion with the Russian government to win the 2016 election—much of which happened while Lewandowski was serving as Trump’s campaign chairman.

Lewandowski ran Trump’s campaign from when Trump announced his candidacy to May 2016, then remained part of the team until late June 2016. Here’s just some of what happened during that stretch:

• Michael Cohen and Felix Sater spent from December 2015 to June 2016 working toward Trump Tower Moscow, a project that involved contacts with high-ranking Russian government officials.
• In April, George Papadopoulos learned that Russia had stolen and planned to publish emails from Trump’s political opponents—making him the first known American to learn of Russia’s plan.
• Paul Manafort and his deputy Rick Gates shared internal polling data and campaign strategy with a suspected Russian spy on the understanding that he would pass the information along to Kremlin-linked oligarchs.
• Carter Page got approval from the campaign to travel to Moscow, where he spoke to multiple Russian government officials about his involvement in the Trump campaign.

Lewandowski needs to answer to Congress about what role he played in these efforts, if any.

• Trump knew—and repeatedly lied—about the Trump Tower Moscow project, and even personally signed a letter of intent. Did Lewandowski know, too?
• Papadopoulos repeatedly updated his superiors on the campaign, including Lewandowski, about his conversations with Kremlin-linked associates. He even sent along several invitations for Trump or other high-ranking campaign figures to travel to Russia. Did he tell Lewandowski about Russia’s plans to release stolen emails?
• Did Manafort have permission from Lewandowski or anyone else to share internal polling data and campaign strategy with outsiders?
• When Page emailed Lewandowski to request permission for travel to Russia, was Lewandowski aware that Russian spies had attempted to recruit Page less than three years earlier?

According to the Mueller report, Lewandowski was also part of one of Trump’s most blatant obstruction efforts.

• In June 2017, Trump decided that he wanted Attorney General Jeff Sessions to un-recuse himself from overseeing the Russia investigation and rein in Mueller. Rather than getting his own hands dirty, Trump had Lewandowski deliver the message (Volume 2, p. 90-93).
• Lewandowski seemed to know what he was doing was wrong. According to Mueller, Lewandowsk said he “did not want to meet at the Department of Justice because he did not want a public log of his visit,” and “stored the notes” from his meeting with Trump “in a safe in his home” (Volume 2, p. 92).
• With repeated prodding from Trump, Lewandowski tried multiple times to get Trump’s message to Sessions, and even deputized Rick Dearborn (then a White House official) to help, but ultimately failed to do so (Volume 2, p. 93).
• Notably, Lewandowski never actually worked in Trump’s administration, which means that even the administration’s absurdly broad claims to executive privilege won’t be enough to block his testimony—although that won’t stop the White House from trying.

Lewandowski likely has important inside knowledge not only about Trump’s efforts to obstruct justice, but also about the collusion Trump was trying to cover up. His testimony will hopefully be an important step forward in exposing the truth about Trump’s corruption and lawlessness.