Source of odor that prompted Center City evacuations remains under investigation

PHILADELPHIA - Officials evacuated hundreds of workers and residents from numerous buildings in Center City due to a strong odor of gas Monday morning. 

Sources had previously indicated that the odor originated from cleaning a tank at the PES refinery in South Philadelphia. 

"PES reports there was a release of mercaptan, a non-hazardous substance, during equipment cleaning," Philadelphia Office of Emergency Management officials tweeted Monday afternoon.  

The Philadelphia fire department now says they cannot confirm the mercaptan release at the refinery as the origin of the odor. 

"After an investigation at PES, the PFD Hazmat Task Force cannot confirm the initial report of a mercaptan release at the refinery. PES did clean equipment today, but the actual source of the smell remains undetermined," the department wrote on Twitter.

While mercaptan is largely harmless, chemical safety experts explain that significant doses can lead to serious health complications and potentially be fatal. 

Several reports have indicated that buildings near 16th and Market Streets all the way up through 19th and Spring Garden Streets had been evacuated. 

Officials from the fire department said they would give updates when available. 

In a tweet, Philadelphia Gas Works claimed that they are not responsible for the gas odors. 

This is a developing story. Stay with FOX29.com for the latest developments. 

For the latest local news, sports and weather, download the FOX 29 News app.

DOWNLOAD: FOX 29 NEWS APP