TIANJIN, China — Thunderstorms Tuesday complicated recovery efforts from last week’s massive explosions at a warehouse in China’s Tianjin port that killed at least 114 people, left 57 missing and exposed dangerous chemicals – including some that could become flammable on contact with water.

Experts have expressed concern that rain could spread some of the vast quantities of hazardous material at the site or set off chemical reactions sparking further explosions. Rain began falling mid-morning, but there was no immediate word of new blasts.

Underscoring the weakness of China’s system of industrial safety, the head of the national organization responsible for those efforts has been placed under investigation for suspected “severe violation of discipline and law” – standard shorthand for corruption – the ruling Communist Party’s anti-graft watchdog said Tuesday.

No details were given about the allegations against Yang Dongliang, who was appointed head of the State Administration of Work Safety three years ago and also worked in state industry and local government in Tianjin for 18 years, including as a vice mayor from 2001 to 2012.

The storms began shortly after residents, firefighters, police, medical staff and officials held a moment of silence marking the sixth day since the disaster, the first observance in the 49-day traditional Chinese mourning cycle. Sirens wailed and car and boat horns blared while assembled groups bowed in respect for the dead.

At the now-evacuated Seaport City housing complex, 33-year-old Fan Jie joined other residents in lighting candles for firstresponders killed in the explosions.

“There were many firefighters who went into the blast and sacrificed themselves. So we’re here today for them, to give thanks to them and grieve for them,” Fan said.

As of Tuesday, 50 firefighters were confirmed killed and 52 others were among the 57 missing, making the disaster the deadliest ever for Chinese firstresponders. About 1,000 firefighters responded to the disaster.

The blasts originated at a warehouse for hazardous material, where 700 tons of sodium cyanide – a toxic chemical that can form combustible substances on contact with water – was being stored in amounts that violated safety rules. That has prompted contamination fears and a major cleanup of a 1.8-mile -radius, cordoned-off area in the port city southeast of Beijing.

Cyanide contamination was found at levels up to 28 times those considered safe at eight of 29 testing sites within the blast zone, said Bao Jingling, chief engineer of the Tianjin Environmental Bureau. No unsafe levels were found at 14 inspection sites outside the zone, he said.

“If the rain gets heavy, water will have to be drained. It is not good for water to remain in the craters,” Bao told a news conference, referring to massive cavities left by the explosions.

Officials have said there have been no substantial leaks of sodium cyanide. They say they have sealed all waterways leading into the sea from the blast site and built retaining walls to prevent any runoff. Sodium cyanide can form a flammable gas upon contact with water, and several hundred tons would be a clear violation of rules cited by state media that the warehouse could store no more than 10 tons at a time.