WASHINGTON- The head of the Federal Aviation Administration (FAA) said Friday he is confident the "root cause" will be determined regarding the crash of a China Eastern Airlines Boeing 737-800.

FAA Administrator Steve Dickson told CNBC that the investigation "requires us to not engage in speculation" in the cause. Dickson added that the 737 NG fleet, which includes the 800, "is one of the safest aircraft ever produced in commercial operation."

Human remains and personal belongings of the 132 people on board have been found but no survivors.

U.S. Transportation Secretary Pete Buttigieg said Wednesday that Chinese authorities had invited the U.S. National Transportation Safety Board (NTSB) to take part in the investigation, adding that he was very encouraged by the invitation to be on the ground in China.

The NTSB, however, later said it had not yet determined if investigators would travel to China in light of visa and quarantine requirements.

The NTSB has said it will support the Civil Aviation Administration of China probe with technical advisers from the FAA, Boeing and engine manufacturer CFM, a joint venture between General Electric and France's Safran.

"By all indications this was an airworthy aircraft but we'll have to go where the facts take us in terms of getting to the root cause," said Dickson, who is stepping down as head of the FAA next week.

Recovery crews on Friday expanded the search area of the crash in China, as the second of the plane's two black boxes eluded search-and-rescue workers for a fourth day.

(Reporting by David Shepardson; editing by Jason Neely)