Boeing CEO Dennis Muilenburg testified before Congress on Tuesday that he was aware of a pilot’s warning that MCAS—the system at the center of two 737 Max plane crashes—was malfunctioning in simulator tests “prior to the second crash.” Mark Forkner, the 737 Max’s chief technical pilot, told a colleague through instant messages that MCAS was making it difficult to gain control of the plane well before it was certified by the Federal Aviation Administration. Congress has been working to nail down what Boeing knew of MCAS’ issues and when.

According to Muilenburg, the company has been unable to speak to Forkner about the messages, despite knowing of their existence for months. Forkner currently works for Southwest Airlines. Lawmakers pressed Muilenburg on why it has taken so long to provide Congress and the FAA with the messages, adding to the mounting criticism over his response to the crashes. Muilenburg told victims’ family members he was “deeply and truly sorry.” “We’ve been challenged and changed by these accidents,” he said. “We made some mistakes, and we got some things wrong.”