Nancy Michael was walking with a friend to a meeting Monday morning along Kellogg Boulevard in downtown St. Paul when they passed what looked to be mounds of dirt piled beneath two oak saplings.

When they looked closer, they were shocked to find that the mounds actually were dead honeybees.

"It looked like small snowdrifts," said Michael. "We went back last night to take pictures, but by then they had swept many of them away."

Some, however, were still hanging in the branches, she said.

It wasn't clear Tuesday what had killed the bees, which may have been a swarm of thousands that had been camping out in the nonflowering trees while searching for a hive box or tree cavity to call home.

But the loss was significant enough that investigators with the state Department of Agriculture's pesticide and fertilizer division visited the site Tuesday morning, removing samples of the bees, tree soil and vegetation for clues on what happened.

Tests were being run but results were not yet available, said department spokeswoman Margaret Hart.

At least 25,000 bumblebees were found dead around linden trees in a Target parking lot near Portland, Ore., last month. Investigators determined that the bees were killed by a pesticide sprayed on the trees to control aphids.

St. Paul spokesman Brad Meyer said he checked with at least three departments Tuesday to see whether any city workers had sprayed the trees, but the answer was negative.

Ramsey County spokesman Art Coulson said the county got a call about bees swarming a couple trees, but it wasn't near county property so they took no action.

The bees were found on the north side of Kellogg near St. Peter Street across from the City Hall and Court House building, outside a vacant former architecture office.

The use of pesticides, along with parasites and disease, is thought to be the main reason why there has been a recent national decline in honeybee populations. Still, a recent federal report on honeybees said that there was insufficient evidence to blame insecticides for the massive losses.