NEW ORLEANS -- 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 the Louisiana Democrat's office couldn't get through to criticize her support of a health care reform bill.

Protesters marched in front of Landrieu's office last December to criticize her support for the 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. 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.