The radioactive material was found in a scrap market
Authorities at Delhi University in India say they are investigating how radioactive waste which this week killed a man was sold as scrap.
On Wednesday, police said cobalt-60 had leaked from an irradiation machine sold by the university earlier this month.
A scrap metal worker who was exposed to the radiation died on Monday of multiple organ failure. Several of his colleagues are still in hospital.
They were exposed to radiation after dismantling the machine, police say.
Search teams have found cobalt-60 in several shops in the scrap market in the city's Mayapuri industrial area.
"Officials from the Atomic Energy Department are investigating the matter. The university is also doing its own investigation with help from the Bhabha Atomic Research Centre," Delhi University Vice-Chancellor Deepak Pental told a press conference.
"The university is very apologetic about what has happened. The university takes moral responsibility for what has happened."
Mr Pental said a thorough investigation should be held so that the "mistake can by rectified".
"We must learn from this incident so that such problems don't recur in future."
The worker who died has been named as Rajendra, who was 35.
On Wednesday, police said the university bought the gamma irradiation machine in 1970 for use in experiments by chemistry students.
The machine, which had not been used since the mid-1980s, was sold at an auction in February.
Police said while dismantling it, the workers peeled off its protective lead cover and in the process exposed themselves to the radioactive metal inside it.
At least half a dozen people were admitted to hospital. Several are critically ill.
The discovery caused panic in nearby residential areas. It was initially thought that the radioactive material had come from hospital waste.
India has a large workforce dedicated to dismantling scrap and experts say it is a major dumping ground for hazardous waste.