In 2004, the FDA was concerned about the potential for "microbial
contamination" of the NECC's injectable products, the letter noted.
"FDA officials could not confirm whether any subsequent inspections were conducted to validate that the … violations were corrected," lawmakers wrote following a briefing with regulators.
Congress has been pushing for action on the outbreak for weeks.
Several members have called for a range of investigations, and introduced bills that would strengthen FDA oversight of operations like the NECC, a compounding pharmacy.
The Framingham, Mass.-based company reprocesses drugs on a large scale to fit patients' needs, such as dividing standard medications into doses fit for children.
As a compounding pharmacy, it is not obligated to follow certain federal guidelines that apply to drug makers.
Wednesday's letter was signed by Energy and Commerce Chairman Fred Upton (R-Mich.) and Ranking Member Henry Waxman (D-Calif.).
Reps. Cliff Stearns (R-Fla.), Diana DeGette (D-Colo.), Joseph Pitts (R-Pa.), Frank Pallone (D-N.J.), Michael Burgess (R-Texas), John Dingell (D-Mich.) and Joe Barton (R-Texas) also signed.