U.S. accident investigators said they remain in the dark about who performed the work on the panel of a Boeing jet that failed in January, despite high-level pleas to the company and interviews at the factory where the fateful removal of the part was performed.

National Transportation Safety Board (NTSB) Chair Jennifer Homendy said she made a direct request to Boeing Chief Executive Officer Dave Calhoun last week, according to a letter to senators on Wednesday. Her team has also been conducting interviews with workers at the company’s Renton, Washington, factory.

Investigators first requested the identity of the crew who worked on a panel that failed on the flight four days after the Jan. 5 accident, Homendy said in the letter. Boeing has said it’s unable to find the records, and factory security footage that may have helped had been overwritten by then.