Chinese authorities said they need more time to determine what caused a China Eastern Airlines Corp. jet to plunge into a hillside last month, offering few new clues to a mystery that resulted in the deaths of 132 passengers and crew.

Investigators are still analyzing data from the two black boxes on the Boeing Co. 737-800 NG aircraft, according to a one-page summary of the report posted by the Civil Aviation Administration of China. No evidence has emerged of any pre-flight mechanical failure, weather issue or problem with the pilots’ qualifications, it said.

The lack of concrete findings after a month of poring over wreckage and other available data suggests that China is being particularly cautious about the investigation and public disclosures.

“After all this waiting, to just have four or five paragraphs that don’t really explain any potential scenario, perhaps that’s by design,” said Jeffrey Guzzetti, the former chief accident investigator at the U.S. Federal Aviation Administration.

Normally, investigators would have been able to determine many clues from wreckage, such as whether engines were functioning at the time of impact or how various equipment was set, Guzzetti said. The summary report included no such findings.

Work to gather data from jet’s two black-box recorders, which were severely damaged in the high-speed impact, is still underway, the report said. Notably, it didn’t say the data was unrecoverable.

Under the Chicago Convention, nations in charge of an investigation must submit a preliminary report to the International Civil Aviation Organization, a United Nations agency, within 30 days of the date of the accident.

Altitude Deviation

The report said the plane entered Guangzhou air traffic space at 2:17 p.m. and that Guangzhou air traffic control then realized it was deviating from the altitude it should have been on. Air traffic controllers radioed the flight deck but received no response, according to the document.

Flight MU5735 from Kunming to Guangzhou fell from the sky about 100 miles from its destination. The last radar recorded from the jet was at 2:21 p.m., the report showed.

The report’s lack of detail drew immediate reaction on social media in China, with some expressing disappointment and frustration. “Nothing’s useful in there,” said one.

While preliminary reports, which may be marked as confidential or remain public at the investigating state’s discretion, contain some information about what was happening on board at the time of the incident, they generally don’t include any formal conclusions as to the cause of the crash. However, they can provide a detailed account of what happened. A full report is due within 12 months.

The main search-and-rescue effort in southern China’s Guangxi region ended late last month and the plane’s black boxes were sent to the U.S. President Xi Jinping called for information about the cause of the crash to come in a timely, accurate and transparent manner, according to earlier Xinhua reports.

A team from the U.S. National Transportation Safety Board and Boeing traveled to China to work on the probe, but returned April 14, the agency said in an emailed statement Wednesday. “The NTSB has an excellent relationship with CAAC,” the agency said in the statement.

Resumed Flights

The jet was flying level at a cruising altitude of 8,900 meters (29,192 feet) when it suddenly dropped, according to the CAAC. Video images of the plane appeared to show it hurtling toward the ground. Data from Flightradar24 indicated it was traveling far faster than normal. The jet “disintegrated” on impact, the CAAC said.

China Eastern resumed flights using the same type of plane over the weekend, a sign the airline is working toward putting the entire fleet back in the air. The carrier is gradually restarting operations for its 737-800 fleet after carrying out checks over the past two weeks, China News Service reported, citing company representatives it didn’t identify.

For now, any scenario that could explain the jet’s unusual, high-speed dive in benign weather remains on the table, Guzzetti said. They include some type of unusual mechanical failure, a pilot miscue or even an intentional act by someone in the cockpit, he said.