DENVER (CBS Denver) — A series of emails appear to be the “smoking gun” in a controversy over the planned expansion of the Colorado Convention Center in Denver, according to multiple sources familiar with the growing scandal.

Four sources familiar with the emails, but not authorized to speak publicly about them, say the documents clearly suggest collusion between Trammel Crow, the company managing the convention center expansion, and Mortenson Construction, a contractor bidding on the $233 million project. One source familiar with the emails told CBS Denver they were “damning.”

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On Tuesday, Denver Mayor Michael Hancock announced a halt to the contractor selection process and said he was asking the Denver District Attorney to investigate.

“And we saw some things that quite frankly concerned us that it was not fair and would lean toward certain players and competitors”, said Hancock.

Trammel Crow said Wednesday it fired an employee but declined to name the person.

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However, multiple sources told CBS Denver the employee was Trammel Crow Senior Vice President of development management Mike Sullivan, 57, who has been with the company since 2006. Those same sources say Sullivan was exchanging emails with personnel from Mortenson and those emails suggest collusion between the two entities.

CBS Denver attempted to contact Sullivan via phone and email but received no response. His presence and past employment with Trammel Crow appeared to have been quickly scrubbed from the companies’ website. Earlier, online bios for Sullivan stated he is a civil engineer from the U.S. Military Academy and has had a more than 20-year career in construction management in Colorado.

In a Thursday afternoon interview, Hancock said he was “dumbfounded” by the emails that underpin the case.

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Click here to read the full report from CBS Denver.