Minnesota U.S. Sen. Amy Klobuchar is questioning Ticketmaster after a Taylor Swift presale event crashed, leaving many fans angry and without tickets.

The Democrat, who chairs the Senate Subcommittee on Competition Policy, Antitrust and Consumer Rights, sent a letter to Ticketmaster on Wednesday expressing "serious concerns" about the lack of competition in the ticketing industry.

"Ticketmaster's power in the primary ticket market insulates it from the competitive pressures that typically push companies to innovate and improve their services. That can result in dramatic service failures, where consumers are the ones that pay the price," Klobuchar wrote.

Two million tickets were sold during the Swift presale event, the platform's highest single-day total, the company said. Fans encountered long queues and errors as they tried to reach the online ticket checkout. Ticketmaster said its site was overwhelmed by real users and bots.

The site crash prompted Ticketmaster to cancel the general public sale it had planned for Friday because it did not have enough tickets left. The company cited "extraordinarily high demands on ticketing systems and insufficient remaining ticket inventory."

In an interview with WCCO-TV, Klobuchar said she will call a congressional hearing on the matter and demanded a Department of Justice investigation. On Friday, national media reported that the DOJ has opened an investigation.

"There just has to be consequences for this type of behavior," Klobuchar told WCCO.

Ticketmaster said in its statement, "we know we can do more to improve the experience and that's what we're focused on."

Swift's 52-date tour, her first since 2018, starts March 17 in Glendale, Ariz., and ends Aug. 9 in Los Angeles.

A range of figures from across the political spectrum joined Klobuchar in criticizing Ticketmaster.

"Daily reminder that Ticketmaster is a monopoly," Democratic U.S. Rep. Alexandria Ocasio-Cortezcq tweeted, saying the merger with LiveNation "should never have been approved. ... Break them up."

Tennessee's Republican Attorney General Jonathan Skrmetti also expressed concern about possible consumer-protection law violations.

Klobuchar has long advocated for more antitrust scrutiny, even writing a book titled "Antitrust: Taking on Monopoly Power from the Gilded Age to the Digital Age."

In her letter to Live Nation Entertainment President and CEO Michael Rapino, Klobuchar said she was skeptical of Ticketmaster's merger with Live Nation in 2011. The Senate held a hearing then on the merger.

"At that hearing, you appeared as a witness and pledged to 'develop an easy-access, one-stop platform that can deliver … tickets.' And you said that you were 'confident this plan will work,' " Klobuchar wrote. "It appears that your confidence was misplaced."

She said the company was subject to an antitrust consent decree after the merger prohibiting it from abusing its market position.

"There have been numerous complaints about your company's compliance with that decree," Klobuchar wrote. "I am concerned about a pattern of non-compliance with your legal obligations."

The senator has asked the company to respond to her letter by Wednesday.

The Associated Press contributed to this report.