GRANDE PRAIRIE, Alta. (CP) - For months the nurses on Unit 4-North of <FORM class=yqin action=http://yq.search.yahoo.com/search method=post>   </FORM>Queen Elizabeth II Hospital have occasionally suffered from unexplained fatigue, dizziness and memory loss.

For months they thought the symptoms might be caused by an air quality problem - maybe sick building syndrome or toxic mould.

But the case has taken a more sinister turn now that medical tests suggest someone is deliberately sneaking tranquillizers into their food or drinks.

"I know we've got a heck of a mystery on our hands," Jim Sanderson, vice-president of human resources for the Peace Country Health Region, said Tuesday.

The recurring symptoms prompted officials to close the unit, which takes up half the hospital's fourth floor, for several weeks while outside environmental consultants conducted a battery of tests on the air and water. They found nothing - in fact they said the air was cleaner than expected in a 20-year-old building.

But when the staff and patients returned to 4-North in the spring, so did the mysterious illnesses. And there was something else.

"We had some other incidents happening, that I'm not at liberty to go into detail about right now, that caused us to involve the RCMP," Sanderson said.

"We are now wondering whether or not the symptoms may be related to somebody's wrongdoing."

He could not say how many nurses were affected, only that there were 20-25 complaints in total. The hospital began to offer diagnostic blood and urine tests to those with symptoms, and in June a few came back positive for a tranquillizer commonly used in the hospital.

The RCMP are now circulating an eight-page questionnaire among the unit staff to try to get to the bottom of it. It focuses on the case of one nurse who was drugged on June 16.

"We have reached the determination that a nurse was drugged purposely in the workplace," reads the first question. "How would you explain this?"

"List the five most important causes that could have created this situation," reads another.

Jerry Macdonald, a nurse at the Queen E and president of the hospital local of the United Nurses of Alberta, said it was "somewhat bizarre occurrences on the unit" that first signalled a possible crime, but he wouldn't go into detail either.

"Some odd things were going on that made it necessary to say to the police, 'Maybe you guys should be involved in checking this out.' "

The whole thing has made the 4-North nurses nervous to say the least.

"Nursing is one of the more stressful occupations out there anyway, without all of this going on," said Macdonald.

Don Brandford of the Alberta Union of Provincial Employees, which also represents hospital staff, agreed the situation is tense.

"I've been doing union business for 15-20 years and I've never ever seen anything like this," Brandford said. "It just baffles me how someone could be doing this to people.

"You should see the rumours flying around there. People are afraid to go to work."

No one has reported any symptoms since last week, when the hospital issued a directive not to leave food or drinks unattended and not to accept food or drink from anyone else.

It means that bringing in a group order from the local Tim Hortons, for instance, is a thing of the past, said Macdonald.

"Now that would be considered unwise. I'm sure that hurts the team spirit."

Over 80 employees, full time and casual, work on the unit. RCMP Cpl. Brent Mundle confirmed that officers are interviewing many of them, but he wouldn't say which direction their investigation was taking. He conceded that it would be "common sense" to conclude the culprit is likely someone who works at the hospital, but police aren't ruling anyone out yet.