Two contract workers injured in the May 3 explosion at International Paper’s Vicksburg mill remained in critical condition this morning at the Joseph M. Still Burn Center in Augusta, Ga., hospital spokesman Beth Frits said.

Robert and Kenneth Townsend were among five people taken to the Georgia burn center after one person was killed and 17 were injured at the Mississippi 3 facility. IP officials have said the blast occurred as workers attempted to restart a recovery boiler during an annual maintenance shutdown.

Two people who have not been identified, but also are believed to be contract workers, are still being treated in Georgia, Frits said.

The Townsends were industrial insulators with M-Co., hired to perform maintenance at the plant. Other unidentified workers injured in the blast are believed to have been employed with other firms hired to do work at the plant.

About 400 people, including IP’s 306 regular employees, were at the mill when the boiler exploded. All were accounted for, company officials have said.

Inspectors with the Occupational Safety and Health Administration are looking into possible causes, with results expected in six months.

In regulatory filings with the Securities and Exchange Commission, IP executives indicated its uninsured losses could not be immediately calculated. Business interruption and property damage insurance held by the company carries $20 million deductibles and retention amounts.

The company also said it is too early to predict a startup date and the explosion’s effect on its work force, about 316 in Vicksburg.

In 2006 and 2007, the company was issued Katrina-inspired Gulf Opportunity Zone bonds totaling $50 million for improvements to its physical plant, including the boilers.