Wednesday night's explosion flattened a pair of row houses and set fire to a block of homes, killing five people - including a 4-month-old boy and a 16-year-old girl.
On Saturday, investigators told The Morning Call of Allentown they were zeroing in on an abnormality in a section of pipe.
Fire Chief Robert Scheirer said workers were to dig up the section of pipe Sunday and take a closer look. The Pennsylvania Public Utility Commission will then send the pipe to a forensics expert.
A spokesman for Reading-based UGI Utilities Inc., says the PUC is running the inspection and will report to the National Transportation Safety Board.