(CNN) — Authorities scanning a remote Australian highway for a tiny missing radioactive capsule have found it by the roadside, after a challenging search likened to trying to find a needle in a haystack.

State emergency authorities announced the discovery on Wednesday afternoon, six days after the capsule, containing highly radioactive Caesium-137, was discovered missing from a package sent hundreds of kilometers from a Rio Tinto mining site in northern Western Australia to the capital Perth.

“Locating this object was a monumental challenge — the search groups have quite literally found the needle in the haystack,” state Emergency Services Minister Stephen Dawson said in a news conference Wednesday.

The radioactive capsule’s disappearance sparked a massive search of the highway with specialized radiation detection units — and prompted warnings to the public not to approach the capsule, which could cause serious burns on contact with skin.

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Authorities believe the capsule — about 8 millimeters high and 6 millimeters round — somehow fell off the back of a truck as it was being transported 1,400 kilometers (870 miles) along the Great Northern Highway from the mine.

Rio Tinto, which had been using the device in a gauge at its Gudai-Darri iron ore mine, said it regularly transports and stores dangerous goods as part of its business and hires expert contractors to handle radioactive materials.

In a statement Wednesday, Chief Executive Simon Trott said the company was “incredibly grateful” for the work undertaken to find the capsule and once again apologized to the community for its loss.

“While the recovery of the capsule is a great testament to the skill and tenacity of the search team, the fact is it should never have been lost in the first place,” he said. “We are taking this incident very seriously and are undertaking a full and thorough investigation into how it happened.”

Authorities said the missing capsule was detected at 11:13 a.m. local time Wednesday, two meters from the road just south of the small town of Newman by crews using radiation detection equipment.

Officials said a 20-meter exclusion zone had been set up around the capsule, and it would be transferred to a lead container before being taken to a security facility in Newman.

On Thursday it would start its journey south again — this time to a health department facility in Perth.

Chief Health Officer and Chair of the Radiological Council Andrew Robertson said it doesn’t appear that anyone was exposed to the capsule’s radiation during the time it was missing.

“It does not appear to have moved — it appears to have fallen off the track and landed on the side of the road. It is remote enough that it’s not in any major community so it is unlikely that anybody has been exposed to the capsule,” he said.

How did it happen?

The Department of Emergency Services WA (DFES) raised the alarm on Friday, alerting residents to the presence of a radioactive spill in the state, including in the northeastern suburbs of Perth, home to about 2 million people.

According to authorities, the capsule was placed inside a package on January 10 and collected from Rio Tinto’s Gudai-Darri mine site by a contractor on January 12.

The vehicle spent four days on the road and arrived in Perth on January 16 but it was only unloaded for inspection on January 25 — when it was discovered the capsule was missing.

The incident came as a shock to experts who said that handling of radioactive materials like Caesium-137 is highly regulated with strict protocols for its transport, storage and disposal.

Radiation Services WA says radioactive substances are transported throughout Western Australia on a daily basis without any issues. “In this case, there seems to be a failure of the control measures typically implemented,” it said in a statement, adding that it had nothing to do with the capsule’s loss.

DFES Commissioner Darren Klemm said the capsule was found in the “best possible area” due to its remote location and to find it in such a short amount of time was “amazing.”

“A lot of work went in around the metro area based on some intelligence early on… so, you can’t help but imagine there was an element of surprise for the people in the car when the equipment spiked,” he said.

Caesium-137 can create serious health problems for people who come into contact with it: burns from close exposure, radiation sickness and potentially deadly cancer risks, especially for those exposed unknowingly for long periods of time.

Robertson, the chief health officer, said that standing one meter from the capsule for one hour would be the equivalent of receiving the radiation dose of 10 X-rays.

Officials had feared the capsule could have become lodged in the tires of another vehicle and transported far from the search zone. It could also have been taken from the area by an animal — or worse, picked up and kept by someone unaware of the dangers.

And the risk wasn’t just in the short-term — Caesium-137 has a half-life of about 30 years, which means that after three decades, the capsule’s radioactivity would halve, and after 60 years would halve again, meaning the lost capsule could have remained radioactive for as long as 300 years.

Robertson said it was unlikely the capsule had contaminated the surrounding soil as it sat unattended for days by the highway.

“It’s encased in stainless steel, so it’s unlikely that, unless there’s been substantial damage to the actual source itself, which is unlikely from a fall from the back of a truck, that there will be any contamination in the area.”

Robertson is investigating the capsule’s disappearance and will submit a report to the health minister in the coming weeks.

Dawson, the emergency services minister, said the capsule’s recovery was an “extraordinary result.”

“I think West Australians can sleep better tonight,” he added.

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