CORPUS CHRISTI — Investigators from two federal agencies have not been allowed near a unit that caught fire last week at Citgo Refining and Chemicals Co. The unit still is considered unsafe.

The alkylation unit continued to burn for more than two days after the July 19 fire started. Alkylation is a key aspect of refining petroleum and Citgo uses hydrogen fluoride, a strong irritant, in that process.

The Occupational Health and Safety Administration sent investigators to determine the cause of the fire. The U.S. Chemical Safety Board sent a four-person team to determine the fire’s cause and evaluate the amount of hydrogen fluoride released.

David Horowitz, a safety board spokesman, said the team had been conducting interviews with witnesses and company personnel but had not been allowed in the unit because of safety concerns.

Rob Hall, the agency’s lead investigator, said it could be another week before the team is allowed in.

“They need to make the unit safe to enter,” Hall said. “It is a very deliberate and methodical process.”

Elizabeth Todd, an OSHA spokeswoman, said her team has been allowed only around the unit’s perimeter as it wasn’t safe to get any closer. She declined to comment further saying OSHA doesn’t comment on open investigations.

Gabriel Alvarado, 34, was injured in the fire and on Monday remained in critical but stable condition at Brooke Army Medical Center in San Antonio. Attempts on Monday to reach family members were unsuccessful.