A religious liberty-focused law firm is calling for an investigation into San Antonio's decision to ban Chick-fil-A from opening up a new location at the city's international airport.

On Thursday, the First Liberty Institute sent a letter to Transportation Secretary Elaine Chao alleging that "religious discrimination" may have been made by members of the City Council of San Antonio during discussions to block the restaurant chain from the revamped Texas airport.

Chick-fil-A was initially included in plans to add over 10,000 square feet of new restaurants and businesses to Terminal A in the airport. The plans were made between the council and Paradies Lagardère, a travel retailer and restaurateur that works with more than 100 airports.

Council members later amended the plans to exclude the Atlanta-based restaurant, citing concerns with its record on LGBTQ issues. The amendment was approved with a 6-4 vote.

"San Antonio should welcome the opportunity to add so popular and successful a restaurant as Chick-fil-A to its airport food offerings, not discriminate against it because the City Council disapproves of its charitable choices," the legal organization wrote in the letter to Chao.

The popular chicken-sandwich franchise's "charitable choices" include $1.8 million donated to groups that discriminate against the LGBTQ community, including the Fellowship of Christian Athletes, according to a ThinkProgress report released the day before the council's decision.

"Councilman Roberto Treviño moved to approve the agreement with Paradies on the express condition that Chick-fil-A be excluded from the contract," the Plano, Texas-based legal firm wrote in the letter.

The letter also called into question  Treviño's public statement calling Chick-fil-A's history riddled with "anti-LGBTQ behavior" and  his suggestion that the city vet all future "economic deals to ensure they align with our core values."

Treviño wasn't the only council member named in the letter.

The legal institute also challenged Councilman Manny Pelaez's statements, saying that he took a significant amount of time to "lambaste, denigrate, and openly mock the otherwise upstanding corporate citizen of Chick-fil-A," during the debate on the pending agreement.

"He described Chick-fil-A as a 'symbol of hate' because it has donated to religious charities that he considered to oppose LGBTQ rights," the letter said.

The First Liberty Institute says that Chick-fil-A was the only restaurant excluded from the concessions deal.

According to              city documents, the new businesses planned for the airport include: Smoke Shack BBQ and Southern Kitchen, Boss Wood Fired Bagels & Coffee, Sip Brew Bar and Market, Local Coffee, the Spurs Store and iStore/Adina's Market.

Last week, Chick-fil-A told USA TODAY that a press release issued by Treviño was the "first we heard of his motion and its approval by the San Antonio City Council."

"We wish we had the opportunity to clarify misperceptions about our company prior to the vote. We agree with the councilmember that everyone should feel welcome at Chick-fil-A," the company said in the statement.

"In fact, we have welcomed everyone in San Antonio into our 32 local stores for more than 40 years."