Some radioactive waste could be missing (Photos: Hindustan Times)
India's atomic energy body has given Delhi University two weeks to explain how radioactive waste, which killed a man this week, was sold as scrap.
The Atomic Energy Regulatory Board (AERB) ordered the university to stop experiments using radioactivity.
A scrap metal worker died on Monday after dismantling an irradiation machine sold by the university.
Meanwhile, a university chemist told the BBC that radioactive waste had been dumped in the campus 20 years ago.
Professor Ramesh Chandra said "more than 20kg...was buried in front of the physics department during 1986-87." He said that his protests at the time were ignored.
A senior official from the Atomic Energy Regulatory Board said its scientists will be checking Professor Chandra's claim - and would decide on whether the campus should be checked for radiation.
Waste disposal rules
Rules governing the disposal of radioactive waste say that the AERB should be informed by the organisation possessing it. The AERB then sends trained professionals to seal the waste and take it away.
"There has been a lapse [by the university in selling the waste to the scrap market]. They should have informed us. We have a strict protocol for radioactive waste disposal," Mr Om Pal Singh said of the irradiation machine .
Delhi University authorities have said they are investigating the incident, described by the AERB as a "serious violation."
On Wednesday, police said that cobalt-60 had leaked from the irradiation machine which was being dismantled in a scrap market in the city's Mayapuri industrial area.
Search teams have found cobalt-60 in several shops in the area.
Police say that others exposed to radiation are still critically ill in hospital.
On Wednesday, police said the university bought the gamma irradiation machine from Canada in 1970 for use in experiments by chemistry students.
The discovery caused panic in residential areas
The machine, which had not been used since the mid-1980s, was sold at an auction in February.
Scientists say that although the radioactive substance in the machine had decayed, it was of high intensity.
Mr Singh said there may have been up to 48 cobalt-60 pencils filled with cobalt pellets in the machine. Some half a dozen pencils have been found by investigators after the incident.
"We are trying to find out how many cobalt pencils were there in the machine and if any are missing," he said.
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.
AERB investigators also found that the cobalt pencils had "got broken and scattered" during the dismantling of the machine.
The discovery caused panic in nearby residential areas.
Mr Singh said that the AERB investigators had carried oit a thorough check of a "large number of shops" in Delhi's Mayapuri area after the incident and had not found any evidence of radioactivity.
Scientists say the incident highlights the lack of safety in Indian research institutes and universities.
India has a large workforce dedicated to dismantling scrap and experts say it is a major dumping ground for hazardous waste.