About a year after China Eastern Airlines Flight MU5735 crashed, killing all 132 aboard, the country’s aviation watchdog said on Monday that investigators were still examining the cause of the accident.

In China’s deadliest aviation accident in 28 years, a Boeing Co 737-800 crashed into a mountainous area in the southern province of Guangxi on March 21, 2022.

The Civil Aviation Administration of China (CAAC) reported that experts had already examined more than 100 pieces of wreckage on the eve of the anniversary.

“However because the accident is very complicated and extremely rare, the investigation is carrying on in an in-depth manner,” the CAAC said.

“Following on this, the technical investigation team will continue to carry out work such as cause analysis, experiments and verification, and will release relevant information in a timely manner according to the investigation’s progress.”

Investigations had focused on how key components were performing before the crash, and looked into air traffic control, the passengers and what the plane was carrying, the regulator added.

Boeing declined to comment and China Eastern did not immediately respond to a request for comment.

A summary of a preliminary report issued last year before the jet’s black boxes were analyzed gave no indication of the focus of the investigation. It noted the crew were qualified, the jet was properly maintained, the weather was fine and no dangerous goods were onboard.

The cockpit voice recorder and flight data recorder were sent to a U.S. National Transportation Safety Board (NTSB) laboratory in Washington, a move Beijing had supported despite rising political tensions between the two countries.

Investigators looking into the crash were examining the actions of the crew on the flight deck, two people briefed on the matter told Reuters last year.

In a response to internet rumors of a deliberate crash, the CAAC said at the time the speculation had “gravely misled the public” and “interfered with the accident investigation work”.

The 737-800 is a widely flown predecessor to Boeing’s 737 MAX and has been in service since 1997 with a strong safety record.

A final report into the causes of the crash could take two years or more to compile, Chinese officials said last year.