1.   Other observers have said the disaster likely was caused by a torpedo explosion.

2.   But what prompted the torpedo explosion remains unclear.

3.   A preliminary report in February said the disaster was caused by a torpedo explosion resulting from the use of volatile fuel.

4.   But Vladimir Kuroyedov was cautious not to pronounce a torpedo explosion as the sole, definitive cause of the undersea tragedy.

5.   Russian navy chief Vladimir Kuroyedov was cautious not to pronounce a torpedo explosion as the sole, definitive cause of the fatal blasts.