(Reuters) - U.S. Senator Elizabeth Warren and three other Senate Democrats on Wednesday sent letters to some of the most prolific SPAC dealmakers, voicing concerns over reports that SPAC insiders exploit regulatory loopholes at the expense of retail investors. Special purpose acquisition companies, or SPACs, are shell companies that raise funds through an initial public offering to take a private company public through a merger at a later date. The lawmakers sought information on how investments in SPACs were solicited, how these executives were compensated and if such compensation was tied to the stock performance of the merged entity created out of a deal between a SPAC and a private firm.We seek information about your use of SPACs in order to understand what sort of Congressional or regulatory action may be necessary to better protect investors,” Senators Elizabeth Warren, Sherrod Brown, Tina Smith and Chris Van Hollen wrote in the letters. The letters were sent to six SPAC investors, including Cantor Fitzgerald’s Chief Executive Officer Howard Lutnick, former Citigroup banker Michael Klein, casino mogul and Houston Rockets owner Tilman Fertitta and veteran investor Chamath Palihapitiya. The letters were also sent to David Hamamoto, who led the SPAC that merged with EV maker Lordstown Motors Corp, and former General Motors Vice Chairman Stephen Girsky. The blank-check market, which saw dealmaking activity at a frenetic pace since last year, has somewhat fizzled out after coming under the scanner of regulators in the United States earlier this year. The U.S. Securities and Exchange Commission is also reviewing such acquisition vehicles, seeking clearer disclosures.