WASHINGTON (CNN) -- Nearly five months before media reports detailed abysmal conditions in parts of Walter Reed Army Medical Center, an internal memo from the commanding officer expressed concern that care provided there was suffering because of the Army's decision to privatize support services.

A September 2006 memo was signed by a deputy to the medical center's commander, Maj. Gen. George Weightman, and was sent to the Army's Medical Command. A copy was obtained by the House Oversight and Government Reform Committee.

It describes in detail concerns about the loss of "highly skilled and experienced personnel" at Walter Reed and the center's increased workload. It concludes that "WRAMC Base Operations and patient care services are at risk of mission failure."

Rep John Tierney, D-Mass., who will chair a congressional field hearing Monday at Walter Reed, told CNN Friday that the decision to "outsource" care from federal employees to IAP Worldwide Services, a private contractor, could have contributed to the deteriorating conditions at the facility.

Tierney also suggested that the decision to hire IAP, despite the company's higher bid for the five-year, $120 million contract, could have been "ideologically driven" -- IAP's CEO is a former Halliburton employee.

Oversight Committee Chairman Henry Waxman, D-California, sent a letter to Weightman on Friday asking him to explain the 2006 memo at Monday's hearing.

It also sent a subpoena requesting him to appear, a step made necessary because Weightman was relieved of his command of Walter Reed on Thursday and, Tierney said, the committee was notified by the Army that it would no longer make Weightman available even though he briefed committee staff earlier this week.

Waxman's letter to Weightman stated that the committee discovered that prior to awarding IAP the contract for support services at Walter Reed there were 300 federal employees working there.

By Feb. 3, when IAP took over the facility, "the number of support personnel had dropped to under 60. Yet instead of hiring additional personnel, IAP apparently replaced the remaining 60 federal employees with only 50 IAP personnel," the letter says.

Waxman also notes in the letter that "IAP is one of the companies that experienced problems delivering ice during the response to Hurricane Katrina."