At least four Indian firms were involved in the manufacture of the components, an official said, but it was still unclear where the contaminated scrap originated.

"We are tracking back the whole chain," Satya Pal Agarwal, head of the radiological safety division of India's Atomic Energy Regulatory Board, told AFP.

"We are taking steps in each place. Exporters have been advised to buy monitors to check their materials before exporting."

France's Mafelec firm delivered thousands of lift buttons to Otis, a subsidiary of the US elevator company, which installed them in at least 500 lifts in the country over the summer.

Otis has said it is now in the process of removing the buttons, after France's Nuclear Safety Authority announced Tuesday that 20 workers who handled the lift buttons had been exposed to excessive levels of radiation.

The French nuclear safety agency has said the lift buttons contained traces of radioactive Cobalt 60.

Swedish officials also said they had found faint traces of radioactivity in steel items imported from India.

The components used by Mafelec were supplied by two Indian firms -- Bunts and Laxmi Electronics -- which purchase inputs from SKM Steels which in turn worked with a foundry called Vipras Casting, Agarwal said.

"The foundries must monitor their input material for any radioactive contamination before smelting," said Agarwal. "Today it happened with Vipras, tomorrow it can happen with someone else."

But Vipras, which says it has purchased radioactivity detection equipment since the incident came to light, told AFP that in this particular case SKM Steels had provided it the steel scrap to convert into bars.

SKM Steels' vice president Girish Chaudhary, who deals with exports, denied that.

"We are not the source of the scrap," said Chaudhary. "We have purchased it from Vipras."

So far, India has not been able to ascertain the source of the contaminated scrap, with hundreds of scrap dealers importing from European countries and the United States, among others.

"We are still completing the investigation then we will be able to tell," said Agarwal.

Demand for scrap has soared because of a boom in construction and manufacturing in India, which imports about 3.1 million tonnes of it a year, but its regulation and inspection regime is often lax.

Four years ago, 10 workers died in a blast at a steel factory while smelting iron scrap from Iran that contained live ammunition.

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