The letter released late Thursday noted that on the day of the battery fire in Boston, someone from Boeing had signed a certification committing it to the NTSB's guidelines for participating in the investigation.
"The NTSB expects parties to an investigation to comply fully with its requirements and respect the role of the agency in performing its investigative responsibilities independently and with transparency," the agency wrote.
The NTSB is leading the investigation into the Boston fire. It is also working with Japanese authorities investigating a smoldering battery nine days later that prompted an emergency landing in Japan.
"We have received the correspondence, and remain fully committed to support the NTSB and other regulatory authorities in their investigations into the cause of the 787 battery incidents," Boeing said in a written statement.
The 787 has been grounded for two months.
Boeing officials said several things at the Tokyo briefing that raised eyebrows. Ray Conner, who runs the company's commercial airplanes division, said Boeing thinks it can wrap up testing soon and get approval to fly the plane within weeks, not months, sounding like Boeing was predicting how quickly the Federal Aviation Administration would let the plane fly again. Conner and other Boeing officials later said that they know the FAA will move at its own pace.