1.   That explosion, in November, was caused by a defective valve on a storage tank, according to Pemex.

2.   The sources said the last telemetry data received seemed to indicate a drop in helium pressure, perhaps due to a defective valve.

3.   The defective valve was found when reactor operations were temporarily suspended in the course of a routine inspection, the officials said.

4.   The last telemetry data received seemed to indicate a drop in helium pressure, perhaps due to a defective valve, he said.