31.   It was not yet known what caused the boat to sink, she said.

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

33.   It is not known what caused the mid-air explosion near Majdal Silim, a Hezbollah stronghold.

34.   It is not known what causes the virus, and there is no cure for it.

35.   It was not immediately known what caused the building to collapse.

36.   It was not immediately known what caused the fire, but firefighters suspect it could have been an electrical short circuit in the air conditioning.

37.   It was not immediately known what caused the van to leave the road it was traveling and crash through the wall.

38.   It was not known what caused the blast on Stonecutters Island.

39.   It was not known what caused the explosion aboard the Evghenia, a water tanker.

40.   It was not known what caused the fire.