CNN  —  Investigators probing the Boeing 737 Max blowout say their investigation is being held back by Boeing’s lack of a paper trail for key work. Despite interviewing employees who work at Boeing’s Renton, Washington facility that assembles the 737 Max, as well as collecting other paperwork, the National Transportation Safety Board says it has not determined who in Boeing’s factory worked on the door plug that left the factory with missing bolts and later blew out on an Alaska Airlines passenger flight in January. Boeing recently said it has searched for records but believes its employees did not document the work. “The absence of those records will complicate the NTSB’s investigation moving forward,” NTSB Chairwoman Jennifer Homendy wrote in a letter to the Senate committee that is also probing Boeing. The letter noted that Boeing has also been unable to provide security footage of the September 2023 work, which included removing and reinstalling the door plug.  Homendy told the Senate Commerce Committee last week that her investigators noticed Boeing “security cameras all over the facility,” but that they were told the footage is kept for only 30 days. The letter revealed that the NTSB’s first request to Boeing for relevant employees’ names came on January 9 — four days after the mid-flight incident. On February 2, the NTSB says Boeing provided “names of individuals who may provide insight regarding the work performed.” NTSB said it requested another list of names on March 2 as it prepared for a series of interviews with Boeing employees last week. In this National Transportation Safety Board (NTSB) handout, an opening is seen in the fuselage of Alaska Airlines Flight 1282 Boeing 737-9 MAX on January 7, 2024 in Portland, Oregon. A door-sized section near the rear of the Boeing 737-9 MAX plane blew off 10 minutes after Alaska Airlines Flight 1282 took off from Portland, Oregon on January 5 on its way to Ontario, California. Handout/NTSB/Getty Images Homendy wrote that the agency is not looking to speak with employees for punitive purposes. “Our only intent is to identify deficiencies and recommend safety improvements so accidents like this never happen again,” she wrote. Homendy signaled that the back-and-forth with Boeing over the names of specific employees is complicating the investigation and said the NTSB would work to protect employees who come forward. “I have become increasingly concerned that the focus on the names of individual front-line workers will negatively impact our investigation and discourage such Boeing employees from providing NTSB with information relevant to this investigation,” she wrote. “To that end, I have instructed NTSB to utilize our authority to protect the identities of the door crew and other front-line employees who come forward with information relevant to the investigation.” Boeing did not have an immediate comment on the letter. CNN has reached out to the committee as well.