U.S. Sens. Brian Schatz and Mazie Hirono have joined a group of senators asking United Airlines to explain what led to Sunday night’s forced removal of a man from a United Express flight.

A letter signed by the senators says “consumer trust and confidence are critical to ensure this industry continues to thrive, and we hope United Airlines will work diligently to immediately address this incident and make necessary improvements to ensure it does not occur again.”

The letter includes several queries, such what is United Airlines’ standard operating procedure when deciding to forcibly remove passengers and how many passengers United has forcibly removed. The letter asks for a response by April 24.

I'm demanding answers from United CEO. Crowdsourcing questions for next letter. Let me know if you have suggestions for round 2. #askunited pic.twitter.com/QSStCIilxv

— Brian Schatz (@brianschatz) April 11, 2017

Twenty-one U.S. senators signed the letter.

“I’m demanding answers from United CEO,” Schatz said in a tweet. “Crowdsourcing questions for next letter.”

Flying @united no longer the friendly skies. Outrageous treatment of passenger, weak response. Needs investigation https://t.co/QgILFPCily

— Senator Mazie Hirono (@maziehirono) April 11, 2017

In addition to the letter signed by Schatz and Hirono, four senators wrote letters today that said United’s explanation “has been unsatisfactory, and appears to underestimate the public anger about this incident.” The letters were addressed to United CEO Oscar Munoz and Ginger Evans, commissioner of the Chicago Department of Aviation.

“The last thing a paying airline passenger should expect is a physical altercation with law enforcement personnel after boarding, especially one that could likely have been avoided,” they said.

The senators directed most of their questions at Munoz, including queries about the airline’s policy for bumping passengers off oversold flights, and whether it makes a difference that passengers have already boarded the plane, as happened on the United Express plane in Chicago.

The senators said the incident could have been prevented with better communication or “additional incentives” — an apparent suggestion that United didn’t offer passengers enough compensation to voluntarily give up their seats.

The letters was signed by the four top-ranking members of the Senate commerce committee — the Republican chairman, John Thune, the Republican aviation subcommittee chairman, Roy Blunt, and the two senior Democrats, Bill Nelson of Florida and Maria Cantwell of Washington.

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New Jersey Gov. Chris Christie says he has asked the Trump administration to suspend regulations that allow airlines to overbook flights.

Christie, a Republican, sent a letter to U.S. Transportation Secretary Elaine Chao on Tuesday, citing a passenger who was dragged off a United Express flight in Chicago on Sunday.

Christie called the practice of “bumping” passengers off flights “unconscionable.”

United is a dominant carrier at New Jersey’s Newark Liberty International Airport.

A message left with Chao’s office was not immediately returned.

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A spokesman for President Donald Trump says it was “troubling” to watch video of a passenger being dragged off of a United Airlines flight.

But White House press secretary Sean Spicer says it’s unlikely the federal government will launch a separate investigation.

Spicer notes that local authorities and United are reviewing the incident in which a man was forcibly removed from a full United Express flight at Chicago’s O’Hare Airport. Videotape of the confrontation spread across social media.

Spicer says he’s sure Trump has seen the video but that any comment from the president could influence a potential outcome of the investigations.

Spicer adds that he thinks everyone who has seen the video can agree that the situation could have been handled better.