TIANJIN, China (AP) — 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.

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 first-responders 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 another 52 were among the 57 missing, marking the disaster as the deadliest ever for Chinese first-responders. 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 3-kilometer (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 the toxic 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.

The Ministry of Industry and Information Technology has ordered authorities at all levels to check whether companies that produce and store hazardous materials comply with safety regulations, including if they are a safe distance from residential areas and do not exceed storage limits.

China's Cabinet, the State Council, said an investigation team headed by Executive Vice Minister of Public Security Yang Huanning has launched a probe into the explosion. Media reports say 10 people have been taken into custody, including top officials of the warehouse's management company.

In unusually strong language, the Cabinet referred to the Aug. 12 blasts as an "especially major fire and explosion incident." Along with the cause, the probe will identify those responsible and provide recommendations on how to deal with them.

Prosecutors previously have said they would investigate any dereliction of duty and abuse of power that may have contributed to the blasts.

More than 40 different types of hazardous chemicals were being stored at the site with a total volume of about 3,000 tons, deputy national fire chief Niu Hueguang was quoted as saying on the China Fire Services' official website.

That included 800 tons of ammonium nitrate, sometimes used in explosives, and about 500 tons of potassium nitrate, often used in rocket fuel and fireworks.

Chinese work safety rules require such storage facilities to be at least 1,000 meters (3,300 feet) away from residences, public buildings and highways. But online map searches show the Ruihai International Logistics warehouse was within 500 meters of both an expressway and a 100,000-square meter (1 million-square foot) apartment complex. Those apartments' walls were singed and its windows were shattered, and all residents have been evacuated.

Some owners of damaged residences have demanded the government buy back apartments that may no longer be habitable due to chemical contamination.

Tianjin officials have been hard-pressed to answer how the warehouse was allowed to operate in its location. Questions also have been raised about management of the warehouse, and the country's top prosecuting office announced Sunday that it was setting up a team to investigate possible offenses related to the massive blasts, including dereliction of duty and abuse of power. Ruihai's general manager is in a hospital under police watch.

The Tianjin blasts are among the deadliest industrial accidents in China in recent years. In June 2013, a fire at a poultry plant in the northeastern province of Jilin killed 121 people. In August 2014, a dust explosion at a metal plant in the eastern province of Jiangsu left 97 people dead.

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Associated Press video journalist Wong Wai-bor in Tianjin and writers Christopher Bodeen, Ian Mader and Didi Tang in Beijing contributed to this report.