RCMP are investigating the construction of the Canada Post mail processing plant, which was built by the company responsible for the Winnipeg police headquarters project that's been under investigation for two years.

Both the airport postal facility and the downtown police HQ were built by Caspian Construction.

Police said in March they hadn't determined whether they would launch a criminal probe into the Winnipeg mail processing plant project, but on Tuesday their response changed.

"The commercial crimes section of the Manitoba RCMP is investigating the construction of the Canada Post mail processing plant located at 1870 Wellington Ave., Winnipeg," RCMP spokesperson Rob Cyrenne said.

Police wouldn't provide further details about the construction of the mail plant near the Winnipeg James Richardson International Airport.

Caspian built the plant as part of a joint venture with Aecon Buildings, based in Toronto.

In 2014, RCMP raided Caspian's offices as part of their investigation into the police HQ and uncovered documents related to the postal plant, according to the report to a justice submitted by investigators after the search.

Police took "two binders containing records and construction records for the Canada Post Winnipeg mail processing plant built by Caspian," court documents said.

"Handwritten notes evidencing fraudulent billing and the payment of a secret commission" were allegedly found by RCMP, the court documents say, along with what police describe as "original and altered subtrades invoices with handwritten notes identifying them as being either 'true' or 'inflated.'"

Construction of the Winnipeg mail processing plant started in 2008 and was finished in 2010. It was billed as the flagship of Canada Post's $2-billion postal transformation project, which involved buying, selling and reconstructing buildings across the country.

Gord Fischer from CUPW wants authorities to get to the bottom of what happened in the construction of the mail processing plant.

The Canadian Union of Postal Workers (CUPW) says there are a lot of unanswered questions.

"I'd like to see a full investigation. I'd like to make sure that in fact that Canada Post, which is owned by the Canadian people is not ripped off," said Gord Fischer, who represents Prairie region CUPW members.

"We are aware of the investigation into Caspian Construction, and continue to fully cooperate with authorities and Canada Post. As this is an ongoing investigation, we are unable to comment further," wrote Nicole Court, Aecon director of corporate affairs in a statement.

RCMP would not specify whether investigators were focused on Caspian Construction's role in the project.

RCMP are investigating the construction of the Canada Post mail processing plant, which was built by the company responsible for the Winnipeg police headquarters project that's been under investigation for two years. 2:07 Canada Post responded to requests for comment by email on Tuesday.

"Canada Post was first contacted by the RCMP on Feb. 2, 2016, regarding their investigation and has been fully co-operating. As this is a police matter, it would be inappropriate to comment further," wrote ​Phil Legault, a Canada Post media relations representative.

Calls to Caspian Construction owner Armik Babakhanians were not immediately returned.