Posters have appeared around Birmingham describing the September 11 hijackers as the "magnificent 19."
Workers from Birmingham City Council are removing the posters
The posters, which have been branded illegal by Birmingham City Council, also feature Osama Bin Laden, the twin towers on fire and advertise a political meeting to be held on the anniversary of the attack.
Birmingham City Council has already removed dozens of these posters but said they had been put up again.
West Midlands Police said that special branch officers are investigating and with the Crown Prosecution Service, are considering whether to bring charges against the organisation behind the posters.
A small radical Islamist group called Al Muhajiroun are featured on the posters.
Al Muhajiroun spokesman Umran Javed said: "For us to air our views with regard to this issue, should in fact fall into the category freedom of speech.
"I don't see how people should have a problem with it.
"We believe what these individuals carried out on September 11 was an extraordinary event."
Gavin Tringham, from Birmingham City Council, said the posters were designed to shock people.
"We're trying to remove them as fast as we can," he said.
"However, they are illegal and we hope to take a prosecution against the person that's put them up and the organisation they're advertising."