The Canadian agency entrusted with tracking radioactive devices that could be used in a terrorist attack provided three different numbers over two weeks when asked how many missing instruments are still unaccounted for.

The confusion raises questions about how closely the Canadian Nuclear Safety Commission is keeping tabs on potentially dangerous tools, several of which experts say could be used to make a dirty bomb.

The commission initially said it knew of just one wayward device in the last few years.

After being challenged, the number climbed days later to 27 since 2002.

The commission said this week that 40 gauges, medical tools and other radioactive devices lost in the last five years are still missing. Of the 40 devices still missing, the commission classified 17 of them as posing a medium safety risk at the time they were lost, and one as high risk. The others were considered low risk.

The Canadian Press domestic news agency challenged the commission's initial responses after compiling its own database of more than six dozen items -- from measuring gauges to electron capture detectors -- that have been lost or stolen, according to the commission's library of incident reports. Those documents were obtained under Canada's Access to Information Act.

The varying figures emerged as anti-terrorism experts and emergency responders warned that even low-level nuclear materials found in gauges, dials and other equipment could be turned into a crude radiological device or dirty bomb.

Rami Jammal, head of nuclear substance regulation for the safety commission, acknowledged it has been a challenge to keep tabs on the gauges, irradiators and other specialized equipment containing potentially harmful materials.

"We must ensure the safety and security of Canadians," Jammal said.

"Unfortunately the world has changed ... You are as strong as your weakest link."

Wesley Wark, a security expert at the University of Toronto, said the commission's inability to monitor its own inventory is alarming and unprofessional.

"The fact that these devices are going missing in these quantities just underscores what I think people in the business know: we haven't yet arrived at a way to fully lock down this material," Wark said.

The commission offered to recheck its numbers when it was pointed out that Inspec-Sol, a Montreal engineering firm, confirmed it lost six gauges to thieves in five incidents between 2004 and 2006.

Radioactive gauges are used when inspecting soils or other materials for radioactive content.

Just one device used to measure soil density was recovered, said company vice president Francois Cote.

The company was penalized with more visits from federal inspectors and an undisclosed hike.

Four of the five thefts involved employee vehicles stolen with gauges locked in trunk boxes, Cote explained. Workers have been repeatedly urged to return such equipment to storage sites at the end of the day instead of keeping them in their cars.

A wake up call came last October when police in Thetford Mines, Quebec, blocked roads in and out of town trying to find a car stolen from the parking lot of a pharmacy while an Inspec-Sol worker was inside the store. A radioactive soil-moisture gauge was locked in the trunk.

Police warned the public against tampering with the potential hazard. It was never found.

In another case, an employee's car was located, but not the missing gauge.

Two more gauges disappeared last December when thieves rammed a truck through the double-locked door of a small Inspec-Sol lab in Tremblant, Quebec. Computers, a fax machine and other equipment were also taken, Cote said.

Cote says Inspec-Sol has spent thousands of dollars to notify the public about missing devices, offer recovery rewards and step up security and training.

"But even if you improve, a car can still be stolen," Cote said.

Of the 35 devices stolen across Canada in the last five years, most were taken from a vehicle or stolen along with the car, truck or trailer, the commission says.

In a typical event, a radioactive darkroom truck was nabbed in late April 2002 from the driveway of a Canspec Group employee in Kitchener, Ontario.

Five years later, it is still missing.

Some 3,200 Canadian license holders, from engineering firms to blood banks, use tens of thousands of sealed radioactive devices in their work.

Federal nuclear inspectors visit license applicants to ensure they will use resources legitimately.