The group behind an ad campaign questioning the official explanation of 9/11, which is running on OC Transpo buses, says its message should be protected as free speech.

The group called Architects & Engineers for 9/11 Truth have launched a global ad campaign called "ReThink911," which is running in other cities including New York and Toronto.

The campaign takes aim at the U.S. government's explanation that World Trade Centre 7 — the "third tower" — fell as the result of fire.

The theory is part of the group's larger beliefs that the collapse of the World Trade Center towers on Sept. 11, 2001, was not the result of a terrorist attack but part of a vast conspiracy and cover-up within the U.S. government.

Carleton University student Andres Acero first spotted one of the group's ads aboard an OC Transpo bus earlier this week. A volunteer with a campus first responders group, Acero said he thinks the ads are disrespectful to the first responders who lost their lives during the 9/11 terrorist attack.

"Take it down. There should be no reason why it's on there. Just take it down. Maybe send an apology to the families of Sept. 11 and all the victims," said Acero.

Transit commission chairwoman Diane Deans agreed, saying while it was a "difficult challenge" to balance the constitutional right to free speech with community acceptability, she thought the ads were "insensitive."

Deans said she would ask for a review of OC Transpo's ad policies at the next transit commission meeting.

Group says campaign sponsored by 9/11 victims

Architects & Engineers for 9/11 Truth responded on Thursday, taking issue with the notion they were being insensitive to run the campaign. They also said their campaign is sponsored by a group representing more than 100 victims' family members.

"To Councillor Deans and to all who question our sensitivity and legal right to run the ReThink911 ads, we would like to make clear: the ReThink911 coalition includes 9/11 victims’ family members who want nothing more than an accurate and unbiased accounting of the death of their loved ones," the group said in a letter published on their website.

Mayor Jim Watson also weighed in on Thursday, calling the ads "disrespectful" but saying he wouldn't ask to have them removed.

"At the end of the day they met the Advertising Standards Council's standards and they are allowed on the buses," said Watson.

"The transit comission can certainly review those and I encourage them to do so. We disagree with the message but you know, free speech is an important principle in our society."