New York City Mayor Michael Bloomberg says a steam pipe explosion in midtown Manhattan was caused by a failure of infrastructure - and not terrorism.

The blast near the Grand Central train terminal Wednesday sent a large column of steam and debris shooting into the air during the afternoon rush hour.

The explosion initially caused fears of a terrorist attack, and panicked workers fled the area.  One person died of a heart attack.  Some 30 other people were injured, some seriously.  

The mayor said the explosion at the 83-year-old steam pipe may have been caused by cold rain water seeping into the pipe.

Tests of debris show the presence of asbestos, a cancer-causing material.  City health officials say so far no air samples have tested positive.

Health officials have urged those who were in the area to isolate their clothing.  Residents and businesses are also advised to keep their windows closed.

Some information for this report was provided by AFP, AP and Reuters.