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.