Pilots of the doomed China Eastern Flight 5735 failed to respond to multiple calls from Chinese air-traffic controllers after tipping into a deadly nosedive, authorities said at a press conference on Tuesday, Bloomberg News reported.

Air traffic controllers received no response to their repeated calls, an official with China’s Civil Aviation Administration said at a press briefing.

The plane disappeared from radar screens at 2:23 pm local time, three minutes after it started a steep descent, the official, Zhu Tao, said at the end of the first full day of searching through the wreckage.

Based on current information, Chinese officials are unable to make a clear judgment about the cause of Monday’s crash, Zhu said. The ongoing investigation will be difficult, he said, describing a devastating impact and hilly terrain in the search area.

The so-called black boxes — separate voice and data recorders which are orange in colour — haven’t yet been found, state-backed news agency Xinhua reported. The team will continue to collect evidence toward a full understanding of the cause, Zhu said.

Investigators are still sifting through evidence to understand why the Boeing 737-800 NG plane carrying 132 people crashed in southern China on Monday.

It’s too early to draw any clear judgments about the cause, Chinese officials said while the dive by the jet from about 29,000 feet is baffling air-crash specialists.

China Eastern has grounded its fleet of 737-800s, and thousands of domestic flights were cancelled Tuesday across the nation even as Boeing offered to help the investigation. Chinese officials ordered a sweeping two-week safety review.

The jet speared into a hillside near Wuzhou in the Guangxi region.

Witnesses said the aircraft disintegrated and emergency teams haven’t found any survivors.

All 123 passengers and nine crew are presumed dead, which would make it China’s first fatal commercial airliner crash since 2010 and the deadliest in nearly three decades.,