The head of the U.S. Chemical Safety Board says agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives and the State Fire Marshal’s Office are blocking his board from investigating the massive April 17 fertilizer plant explosion in West.

Rafael Moure-Eraso, chair of the Chemical Safety Board, has appealed to Senator Barbara Boxer (D – Calif.) for aid in gaining access to crucial evidence he says the ATF has withheld from CSB investigators. Boxer is head of the U.S. Senate Committee on Environment and Public Works and has announced plans to hold hearings on the West explosion.

In a May 17 letter to Boxer, Moure-Eraso wrote:

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.

Moure-Eraso says the ATF and Fire Marshall’s office blocked CSB investigators from the scene of the blast for three weeks, during which time they drastically altered the site with excavation equipment as part of their ultimately fruitless investigation into the cause of the fire that triggered the explosion.

A spokesperson for the Fire Marshall’s office says CSB investigators were denied access to the blast site because criminal investigators were executing search warrants.