The Carroll Junior High School choir was halfway into its third song - "Carol of the Bells" - during a Christmas performance at Carrolton Elementary School yesterday when one of the singers standing on the top step of their platform felt dizzy.

The singer climbed down and quietly left the stage, just as the choir's instructor had said to do if anyone felt ill.

The performance continued as a second student left the top step, then a third and a fourth. One student lost consciousness and had to be lowered to the floor by two fellow singers.

By the time "Rudolph the Red-Nosed Reindeer" concluded the 20-minute performance, 15 of the 64 singers had disappeared from the stage, according to Maureen Batson, the choir instructor.

The Prince George's County Fire Department was called and several ambulances took the 15, who were suffering from dizziness, loss of facial coloration and nausea, to Doctor's Hospital in Lanham. They were all treated and released.

At first, officials at the school in New Carrollton believed the students were just over-excited and that the heat in the school's multipurpose room contributed to their malaise.

Then the fire department, using special equipment, detected an "abnormal gaseous situation" in the room.

The school was immediately evacuated and school personnel along with the fire department, the Prince George's County Health Department and the Washington Gas Light Co. began a hunt for a source of leaking gas.

At first, the investigators were puzzled because only choir members on the upper risers were affected rather than the crowd of elementary students who sat nearby.

This prompted the health department investigators to check the junior high school cafeteria where they thought they might find a possible cause of food poisonous for the choir members. The cafeteria, which serves breakfast, had no signs of contamination.

Next, gas company officials reported there was some carbon monoxide in the area near the elementary school cafeteria kitchen.

They said the stove near the room where the students performed might have malfunctioned and released carbon monoxide. This explanation was abandoned when gas company investigators were unable to find a malfunction in the stove or enough carbon monoxide in the immediate area to cause the illness among the choristers.

Next, a deflated basketball was discovered in the flue of the school's hot water heater.

And there the mystery ended.

The basketball blocking the flue had caused incomplete combustion that produced carbon monoxide. School officials said the singers on the highest risers were affected first because the gas is lighter than air and rose above the others.

The elementary students were sent home following yesterday's incident. Carrollton Elementary principal Marylouise Callahan told a reporter the school will reopen Monday.