Many observers were surprised to learn, in a New York Times report, that the contractor who destroyed Hillary Clinton's emails while they were under a congressional subpoena received immunity from the Justice Department — and then still refused to answer some questions from the FBI. The surprised included the top two Clinton email investigators in Congress, Reps. Trey Gowdy and Jason Chaffetz, who have both pored over the Clinton materials the FBI handed over to lawmakers.

It was news to them. Chaffetz, chairman of the House Oversight Committee, had no idea until the Times learned it. Same for Gowdy, head of the Benghazi committee.

"If there is a reason to withhold the immunity agreement from Congress — and by extension, the people we represent — I cannot think of what it would be," Gowdy said in a statement. "I look forward to asking the Bureau about any witnesses who were granted immunity or claimed a privilege preventing them from answering questions."

Chaffetz will start asking those questions tonight — in a rare Monday evening hearing before the Oversight Committee. Among the witnesses called are top officials who deal with Congress for the FBI and the Justice Department.

Perhaps the key question Chaffetz and his fellow Republicans — Democrats won't be interested — will pose is: What more have you not told us? Committee members appreciated the FBI handing over Clinton email investigative materials, but they have since learned that the FBI was selective in what it gave to Congress. There are documents that weren't handed over, and there are significant blackouts in the documents that were given to lawmakers.

The most fundamental questions concern the extent of the classified information that Clinton mishandled. In his July 5 public statement, FBI Director James Comey revealed that 110 of Clinton's emails had material that was classified at the time they were sent, and more than 2,000 additional emails had material that was later determined to be classified. But what was the classified information that Clinton mishandled? Of course the FBI cannot release it, but it could give Congress, and by extension the American people, a more precise characterization of the secrets Clinton handled in what Comey called an "extremely careless" way.

And then there is the process of the FBI's investigation itself. It's not just the fact of the email deleter's immunity that Congress didn't know. Anyone who has had a chance to look over the only two documents that have been released to the public — a heavily-redacted copy of the FBI's summary report plus the so-called "302" writeup of the agents' interview with Clinton — has come away with some very important questions unanswered.

Like: Who actually gave the order to delete Clinton's emails? And: Did that order come after — not before — that material was under a subpoena from Congress? And: Who decided to give the deleter immunity? And: What questions did the deleter refuse to answer? And: What does immunity mean if the witness granted immunity still refuses to answer questions? And more.

That's what the Oversight Committee's second hearing, to be held Tuesday morning, will address. Called to testify — actually subpoenaed to testify — will be Paul Combetta, the technician for Platte River Networks, the Colorado contractor hired by Clinton to handle her email system. Combetta is the man who had the infamous "oh s—t" moment described in the FBI report in which he allegedly realized he had forgotten to carry out a December 2014 order to destroy emails and instead carried it out sometime in March 2015, after the material was under congressional subpoena.

Combetta is also the one who was given immunity and then, according to the FBI, refused to answer questions about a conference call he had around the time of the deletions with Clinton lawyer David Kendall and top Clinton operative Cheryl Mills.

Republican investigators have a lot of questions for Combetta. The first is whether he will show up. Right now, that's not clear.

Also called to testify is Bryan Pagliano, the former State Department tech worker who set up Clinton's private email system — and who has also received immunity from the Justice Department. Along with them will be another worker for Platte River Networks and a longtime employee of former President Bill Clinton.

Pagliano's immunity has been known for a while. But the revelation of immunity given to Combetta, who the FBI report says destroyed Clinton's emails and backups after the subpoena, deeply troubles Gowdy, a former prosecutor.

"They gave immunity to the trigger man," Gowdy told Fox News on Friday. "I mean, that's why those of us who used to do it for a living didn't like to give immunity ... They immunized the one person you most want to prosecute for the destruction of government records."