PONTIAC, Mich. – The federal government is investigating a rash of deaths at a United States Postal Service (USPS) facility in Pontiac which may be connected to methane gas leaching from the ground.

Five people have died recently at the USPS Michigan Metroplex Processing and Distribution Center on East Montcalm Street. There is concern that methane gas leaching from the ground into the plant has led to the deaths.

At 70 acres, the Metroplex facility is one of the nation's largest. Built on the site of the old Brownfield Foundry, it opened in 2008 and employs about 1,200 people who work three shifts a day, 24/7.

A seven-page internal report by the USPS inspector general's office explains concerns about underground methane. The report includes a picture of a methane monitoring system built into the building but not working anymore.

"This review was initiated based on safety concerns expressed to us in a complaint which identified five employee deaths during a 14-month period," the report reads. "We have not correlated these deaths to the safety concern at this time, but are continuing work in this area. However, we are reporting this issue with the methane detection system for your immediate action. The MDS at the Metroplex has not been functioning properly since March 2015."

The plant's workers moved to the Metroplex from five different Troy buildings. Long-time employee Steve Western pines for the old days.

"It just became a horrible place to work," said Western. "We're consolidated in one facility. In my 22 years there's never been no deaths. Since we got here four or or five people have passed away."

Mail Hauler's Union Local President Marlon Harris is deeply concerned about his members' recent on-the-clock deaths.

"Three (deaths) they are telling me were heart attacks. We had one girl who walked out having an asthma attack, and she passed out in her car. There's another guy who had a stroke," said Harris.

The Postal Service said the deaths all were due to natural causes. They have had the methane levels in the plant tested and claims the levels are safe.

Postal Service spokeswoman Elizabeth Najduch released the following statement Wednesday afternoon:

"In the management response to the OIG report, the Postal Service indicated that it remained committed to the health and safety of its employees. While the Postal Service agreed with the recommendations in the OIG’s report, the Postal Service emphasized that there was never any health or safety threat to employees at the Michigan Metroplex. As specifically noted in the OIG’s report, a third party vendor conducted an independent evaluation on January 15, 2016 and found no immediate health or safety concerns regarding methane levels at the Michigan Metroplex. The Postal Service did agree to evaluate and resolve any issues related to sensors on its methane detection system but again referenced the independent evaluation confirming there was no safety threat to employees."

Here is the full report from the USPS inspector general's office: