Investigators on Thursday expanded the search area for the second black box from a China Eastern passenger jet that crashed in southern China with 132 people on board earlier this week.

Off-and-on rain was impeding the search for a second straight day. One of the black boxes, believed to be the cockpit voice recorder, was found on Wednesday. Its outer casing was damaged but the orange cylinder was relatively intact, investigators said.

Also read: What is airplane black box and how does it work?

The Boeing 737-800 was cruising at 29,000 feet (8,800 metres) when it suddenly nose-dived into a remote mountainous area on Monday, setting off a fire in the surrounding forest that could be seen in NASA satellite images. No survivors have been found.

The Associated Press reported that searchers are using hand tools, metal detectors, drones and sniffer dogs to comb the heavily forested and steep slopes. Wallets, identity and bank cards and human remains have been found.

Also read: China plane crash: One ‘black box’ found in severely damaged condition

News organisation CCTV news showed police in olive- and dark-colored rain gear standing in a cleared area on Thursday, some with bare or white-gloved hands atop long-handled tools that appear to be shovels or sickles. They all wore surgical masks.

Recovering the so-called black boxes — they are usually painted orange for visibility — is considered key to figuring out what caused the crash.

Also read: Explained: What are the latest clues in China’s plane crash?

Cockpit voice recorders can capture voices, audio alerts and background sounds from the engine or even switches being moved. The flight data recorder stores information about the plane’s airspeed, altitude and direction up or down, as well as pilot actions and the performance of important systems.

Investigators have said it is too early to speculate on the cause. “An air-traffic controller tried to contact the pilots several times after seeing the plane’s altitude drop sharply, but got no reply,” officials have said.

Also read: China plane crash: Search operations suspended as rain washes away debris

The China Eastern flight was headed from Kunming, the capital of Yunnan province, to Guangzhou, a major city and export manufacturing hub on China’s southeastern coast. China Eastern, headquartered in Shanghai, is one of China’s four major airlines.