NEW ORLEANS (AP) — Four conservative activists accused of trying to tamper with the phones in Sen. Mary Landrieu's office pleaded guilty Wednesday to misdemeanor charges of entering federal property under false pretenses.

James O'Keefe, 25, famous for wearing a pimp costume in a video that embarrassed the community organizing group ACORN, was sentenced to three years probation, 100 hours of community service and a $1,500 fine.

The FBI has said O'Keefe used his cell phone to try to capture video of two others who posed as telephone repairmen and asked to see the phones at Landrieu's office. O'Keefe has said the group was trying to investigate complaints that constituents calling Landrieu's office couldn't get through to criticize the Democrat's support of a health care reform bill.

Protesters marched in front of Landrieu's office in Baton Rouge last December to criticize her support for the health care legislation and complained they couldn't get through on her office phones. Landrieu said at the time that her office was receiving a high volume of calls and apologized.

O'Keefe told the judge he regretted his actions and apologized for raising security concerns at the federal building. After the hearing, he said he would continue his undercover work.

"What I do is I stand up to power," O'Keefe said. "I expose corruption in the back rooms."

Magistrate Daniel Knowles III sentenced the three other suspects to two years probation, 75 hours of community service and $1,500 fines.

O'Keefe, Stan Dai, Joseph Basel and Robert Flanagan were arrested Jan. 25 on felony charges, but federal prosecutors later reduced the charges.

Knowles chided the defendants, saying they came from good backgrounds, all had undergraduate degrees or better, and should have used better judgment.

The judge told O'Keefe he was particularly concerned about his role as ringleader.

"One of the things involved in being a journalist is you're going to have to learn to draw the line," Knowles said. "In this instance, you drew it in the wrong spot."

O'Keefe said he would release the results of another such investigation very soon.

"This is about sunlight and we're the going to be the disinfectant," O'Keefe said without answering any questions.