DENVER -- The U.S. Secret Service on Monday said it was investigating the claims of three people who said they were removed from President Bush's town hall meeting on Social Security last week after being singled out because of a bumper sticker on their car.

The three said they had obtained tickets through the office of Rep. Bob Beauprez, R-Colo., had passed through security and were preparing to take their seats when they were approached by what they thought was a Secret Service agent who asked them to leave.

One woman, Karen Bauer, 38, a marketing coordinator from Denver, said the agent put his hand on her elbow and steered her away from her seat and toward an exit.

"The Secret Service had nothing to do with that," said Lon Garner, special agent in charge of the Secret Service district office in Denver. "We are very sensitive to the First Amendment and general assembly rights as protected by the Constitution."

The Secret Service is in charge of protecting the president.

The three who were removed, along with their attorney, Dan Recht, met with Garner Monday. Recht said he may file a lawsuit based on the group's alleged violation of their First Amendment rights.

Garner said the group appeared confused as to who asked them to leave and declined to release further details, citing an ongoing investigation.

Alex Young, 25, an Internet technology worker from Denver who was among the three removed from the event March 17 at Wings over the Rockies, said officials told them the next day they were identified as belonging to the "No Blood for Oil" group.

Young said they belong to no such group, but the car they drove to the event had a bumper sticker that read: "No More Blood for Oil."

"I don't think a bumper sticker on a friend's car should disqualify me from seeing the president," Young said.

Beauprez distributed tickets to the event, which was part of President Bush's effort to gain support for his plan to overhaul Social Security. Messages left after-hours at Beauprez's office were not immediately returned Monday. A call to Bush's advance team in Denver went unanswered Monday afternoon.

Lawrence Pacheco, a spokesman for Rep. Mark Udall, D-Colo., said the congressman had asked the Secret Service about the group's allegations.

Young, who along with Bauer and lawyer Leslie Weise, 39, is a member of the Denver Progressives, a political activist group. He said the three had T-shirts underneath their business attire that read, "Stop the Lies" and they had talked about exposing them during Bush's visit. He said they had scrapped the plan by the time they arrived at the museum.

Recht said the T-shirts did not play a role in the group's removal.

"They hadn't done anything wrong. They weren't dressed inappropriately, they didn't' say anything inappropriate," Recht said. "They were kicked out of this venue and not allowed to hear what the president had to say based solely on this political bumper sticker.

"The very essence of the First Amendment is that you can't be punished for the speech you make, the statements you make," Recht said.

Several high profile groups, including the older citizens' lobby AARP, oppose changes to Social Security that would privatize the country's retirement safety net.

President Bush has visited at least 17 states since the State of the Union to gain support for his plan, meeting with people who are generally supportive.

Some people who have stood up to disrupt Bush while he was talking have been removed. But a group called Americans United to Protect Social Security said there have been at least two additional instances where people who have done nothing wrong have been removed or barred from a Bush event beforehand.

One instance happened in February in Fargo North Dakota, where a "black list" of people banned from getting tickets was obtained and published by the Forum of Fargo-Moorhead. The White House and the Republican Party denied such a list existed and Gov. John Hoeven's staff said nobody was denied tickets.

Brad Woodhouse, a spokesman for Americans United, called the Denver example the most egregious violation.

"They're screening the people who are allowed to come and then they're profiling them in the parking lot," he said. "It's quite extraordinary, and disappointing."