Lawyers for an investigative reporting organization filed a lawsuit Wednesday against the Philadelphia region's main transit agency after it refused to run ads about the group's stories on racial disparities in mortgage lending.

The American Civil Liberties Union, along with co-counsel representing the Center for Investigative Reporting, filed the federal lawsuit against the Southeastern Pennsylvania Transportation Authority over a ban on advertising involving "matters of public debate," saying the center's First Amendment rights were violated. SEPTA denied the center's January request to buy ad space on buses to run information from an investigation on racial disparities in mortgage lending practices.

"SEPTA as a governmental entity should not be restricting speech, and time and again courts have thrown out similar efforts by the government to regulate speech in forums such as this," said attorney John S. Stapleton, co-counsel with the ACLU.

A spokesman for SEPTA said the agency's advertising restrictions are constitutional and the center's claim is inaccurate.

"SEPTA's decision is consistent with its advertising policy, which is constitutionally sound and necessary to ensure that our vehicles and stations do not become forums for political debate — and distract from our core mission of providing safe and reliable public transportation in an environment that is welcoming to customers and employees," said spokesman Andrew Busch. "To protect this critical public service, SEPTA will vigorously defend against this lawsuit."

The lawsuit says the center wanted to advertise on buses in the neighborhoods most affected by their findings, but there is a lack of opportunities for effective advertising. Since SEPTA has declined the ads featuring comics that offer people a way to find out more about the center's investigation and findings, the center has contracted with the city of Philadelphia to run ads on some of its spaces. Stapleton said he wasn't sure where those advertisements would be.

SEPTA changed its advertising policies to prohibit all political speech after a 2015 lawsuit by the American Freedom Defense Initiative over the agency's refusal to run ads it deemed anti-Islamic. A federal judge ruled the agency must run the ads that featured a 1941 photograph of Adolf Hitler with a former Arab leader and the tag line "Jew Hatred: It's in the Quran."

The ads ran on about 84 buses for a month after the judge's ruling.

SEPTA added about six new restrictions on advertising after the ruling, including a ban on political advertising. The center's attorneys say it is too broadly defined.

The rules prohibit ads promoting or opposing a political party, election, candidate, or contain a political figure or issue that is "political in nature," or expresses opinions on matters of public debate including historical, religious and social issues.

In a statement released by the ACLU, the center's editor in chief called SEPTA's ability to define factual information as political advocacy a "dangerous slope."

"Getting important information into the hands of those most affected is a key tenet of both journalism and government," Amy Pyle said.

The Associated Press published the center's story on mortgage lending disparity after it independently reviewed and confirmed the data.