Federal safety investigators are examining a natural gas pipeline for fractures and other damage as they gather evidence on the cause of last week's deadly explosion at a Pennsylvania chocolate factory, a spokesperson said Wednesday.

The National Transportation Safety Board opened a probe into Friday's blast at R.M. Palmer Co. that killed seven people, wounded several others and leveled the building in West Reading, a small town about 50 miles northwest of Philadelphia. Autopsies preliminarily revealed that all seven died of blast injuries, the coroner's office said as it released the victims' names.

Federal investigators have said natural gas was involved in the explosion.

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"NTSB is continuing to gather evidence about how the building was supplied with natural gas and point of ignition, interview witnesses, examine the pipeline for fractures, any damage to pipeline, a chronology of events leading up to the explosion, among other issues that may come up as the investigation continues," agency spokesperson Keith Holloway said by email Wednesday.

A preliminary report on the explosion could be available in about three weeks, whereas the final report could take up to two years, he said. Pennsylvania State Police are also investigating the cause.

Pennsylvania chocolate factory explosion

Federal investigators are searching natural gas lines for fractures at the site of a deadly factory explosion in eastern Pennsylvania. (AP Photo/Michael Rubinkam)

Some workers had told relatives they smelled natural gas before the blast, although the gas utility, UGI, said it had received no reports of a gas leak. UGI said it is cooperating with the probe.

The family that owns the candy company said in a statement Wednesday that the loss of their employees "will be felt forever."

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"My family and I are deeply saddened and heartbroken by the devastating loss of several colleagues and friends as a result of last week’s tragic incident at the West Reading facility," said the statement from Richard Palmer Jr. and his family.

The Berks County coroner's office on Wednesday released the identifies of five of the people who were killed: Xiorky D. Nunez, 30, of Reading; Susan H. Halvonik, 63, of Upper Providence Township; Michael D. Breedy, 62, of Marion Township; Diana M. Cedeno, 44, of Reading; and Judith Lopez-Moran, 55, of Reading.

The names of the other two people killed, Amy S. Sandoe, 49, of Ephrata, and Domingo Cruz, 60, of Reading, were previously released.

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All seven victims appear to have died of blast injuries, Berks County Coroner John Fielding said at a news conference. The manner of death is pending further investigation by other agencies, he said.