To avoid mass panic, Nanjing authorities delayed for a day and a half before informing residents that deadly iridium-192 was loose in the East China city.

As four Tianjin Hongdi Engineering Development Company company employees were criminally detained for their alleged role in losing the radioactive isotope on Sunday, concerns were being raised about the necessity for a 36-hour delay mentioned by the environmental bureau in its official microblog.

Lost on Wednesday, the iridium-192 was found in grass one kilometer from its original location, then wrapped in a plastic bag and placed in a lead-lined pot 5:30 pm on Saturday, China Central Television reported.

The bag indicated someone might have had contact with it, the station said on Sunday.

Iridium-192 the size of a soybean is so deadly that if it comes into contact with skin, amputations are required, experts say.

Company workers first noticed the loss at a Sinopec-owned yard on Puliu Beilu Road on Thursday night and reported it about midnight, the bureau said.

The information was not released to the public until noon on Saturday. On Saturday afternoon, the substance was found and potted.

The delay in releasing information suggested a "dereliction of duty," Zhu Lijia, a professor of the Chinese Academy of Governance, told the Global Times on Sunday. "Late release of information only makes people more doubtful about the government's ability to handle a crisis.

"Issues of public safety especially those involving radioactivity which highly concern the public should be publicized in good time with full transparency about the government's counter-measures."

Sinopec site workers received a physical examination on Friday, which excluded the possibility of accidental radiation injury and radiation substance diffusion, the bureau said.

"It was an enclosed plant with only one gate and active guard and surveillance which could prevent the free flow of people," the bureau announced on its microblog Sunday.

Experts believe the human body would not be harmed farther than 30 meters away, the release said.

Whoever carried that stuff should get a blood test at the hospital, said nuclear safety expert Gui Liming of Tsinghua University in Beijing.

The loss implied serious management deficiencies, Gui told the Global Times on Sunday, but there should be no further damage to the surrounding environment.

"It should be kept in the pot and only taken out using a special device," he said. There must have been an operator error if the substance was found on the ground, he said.

Zhang Qi contributed to the story