Safety Board: ATF Blocking Plant Blast Probe Federal agents and the state fire marshal have effectively barred a federal safety panel from the site of a Texas fertilizer plant blast that killed 15 people and injured about 200 others, hampering its investigation, the panel's chairman said. In a May 17 letter to Sen. Barbara Boxer, U.S. Chemical Safety Board Chairman Rafael Moure-Eraso asked the California Democrat to help the board obtain evidence under control of the Bureau of Alcohol, Tobacco, Firearms and Explosives that he contends is essential to the board's investigation, the Waco Tribune-Herald and Austin American-Statesman reported. "To date, the CSB has experienced significant obstacles that potentially compromise and delay our ability to complete the 'comprehensive investigation' that you have rightly demanded, and that we would very much like to produce," he wrote to Boxer. The chairwoman of the Senate Committee on Environment and Public Works has said she planned to hold hearings into the April 17 West Fertilizer explosion. A criminal investigation "comes with certain sensitivities. You need to keep it to law enforcement only," Robert Champion, ATF special agent in charge of the investigation told the American-Statesman. He also said the decision to bar the CSB from the site was made by the State Fire Marshal's Office. Fire Marshal's Office spokeswoman Rachel Moreno said the CSB was kept out because criminal investigators were executing search warrants. "We have to protect evidence," she told the American-Statesman. We need to have one report, one set of interviews; it all has to be clear cut. Messages left by The Associated Press with ATF, the State Fire Marshal's Office and Boxer's Washington office were not returned on Tuesday night. In an April 30 statement, Boxer said she "cannot rest until we get to the bottom of what caused the disaster" in West and that she wants to make sure such facilities are complying with chemical safety laws. In his letter, Moure-Eraso said the board sent 18 investigators and other experts to West within 24 hours of the blast. At the same time, ATF "assumed essential exclusive control of the incident site" with the State Fire Marshal's Office, he wrote. "These criminal investigators have exercised exclusive control of the site for a full one-month period ... and have altered or removed almost all relevant physical evidence at the site," he wrote. ATF and the State Fire Marshal's Office "consistently expressed the position that CSB was not permitted to conduct separate interviews, prepare expert analysis or author its own independent report," he wrote. ATF and the state fire marshal "state that because in their view this was exclusively a criminal investigation, there could be only one version of what occurred and one report." On May 16, representatives of the State Fire Marshal's Office announced that the joint criminal investigation left the cause of a fire precipitating the blast as "undetermined." Investigators narrowed the number of possible causes to three: a problem with one of the plant's electrical systems, a battery-powered golf cart and a criminal act. However, they could not say with certainty what caused the fire that ignited stored ammonium nitrate, said Kelly Kistner, the assistant state fire marshal.