1.   Although it is not yet known what caused the death of Boston Ballet dancer Heidi Guenther, she was far from alone in her quest for thinness.

2.   But Larson said it might never be known what caused the two park workers to fall.

3.   It is not known what causes the disease and there is no cure.

4.   No one knows what causes these late effects, how many will suffer them, or what to do about them.

5.   Police said it was not immediately known what caused the massive explosion, which occurred near a footbridge on the six-lane highway.

6.   Although police said they did not know what caused the explosion, fire fighters said they found burned out propane gas canisters soaked in gasoline where the fire started.

7.   He said it was not known what caused the tree to fall.

8.   He said it was not immediately known what caused the accident at the building site for a four-story clubhouse.

9.   It is not known what caused the failure of the probe, considered a devastating blow to the troubled Russian space program.

10.   It is not known what caused the power cables to fail, and a commission of experts says it may take six months to a year to find out.