Landrieu's spokesman called his explanation "feeble."
O'Keefe, known nationally for hidden-camera videos targeting the community-organizing group ACORN, said he believes it's clear he and others weren't trying to wiretap or shut down Landrieu's phones in her office in a New Orleans federal building.
He said the four, including two who posed as telephone repairmen, wanted to investigate criticisms that people Landrieu represents could not reach her office by phone.
"I learned from a number of sources that many of Sen. Landrieu's constituents were having trouble getting through to her office to tell her that they didn't want her taking millions of federal dollars in exchange for her vote on the health care bill," O'Keefe said in the statement.
"I decided to investigate why a representative of the people would be out of touch with her constituents for 'weeks' because her phones were broken. In investigating this matter, we decided to visit Sen. Landrieu's district office -- the people's office -- to ask the staff if their phones were working."
Some have criticized Landrieu for supporting a Senate health care reform plan and striking a deal to secure a Medicaid provision with an estimated value of as much as $365 million for Louisiana. The provision was meant to address what state officials, Republican and Democrat, agree is a problem: Louisiana faces what they say are unfair cuts in Medicaid based on state income figures temporarily inflated as money flowed into the state following Hurricanes Katrina and Rita in 2005.
Landrieu said through a spokesman Friday that she wants to see the outcome of the federal investigation.
"It is obvious to anyone following this case that James O'Keefe crossed the line and Sen. Landrieu expects a thorough federal investigation into the matter," spokesman Rob Sawicki said. "She believes that he should save his feeble explanation for the FBI and the judge."
Landrieu responded in December to complaints about phone problems in her office, saying a flood of calls jammed lines. Her spokesman has denied anyone on staff intentionally ignored or mishandled calls.
O'Keefe and the others face a charge of entering federal property under false pretenses for the purpose of committing a felony, which carries up to 10 years in prison. They are free on $10,000 bail.
O'Keefe must remain with his parents in Westwood, N.J., and can travel only after obtaining court approval, according to the conditions of his release.
Charged along with him are Robert Flanagan of New Orleans, Joseph Basel of Minnesota and Stan Dai of the Washington, D.C., area, all 24. The four are due back in court Feb. 12.
"The sole intent of our investigation was to determine whether or not Senator Landrieu was purposely trying to avoid constituents who were calling to register their views to her as their senator," O'Keefe said in his statement Friday. "We videotaped the entire visit, the government has those tapes, and I'm eager for them to be released because they refute the false claims being repeated by much of the mainstream media."
Basel said he concurred with O'Keefe's statement but could not comment further.
"Understand that because of the seriousness of the allegations we cannot speak to the case or the charges basically beyond what James wrote in that statement at this time," Basel said in a phone call to The Associated Press in Minneapolis.
O'Keefe last year became famous for his videos about ACORN, the Association of Community Organizers for Reform Now, which has affiliates that register voters in urban and other poor areas of the country. He used a hidden camera to record as he brought a young woman posing as a prostitute to the group's offices.
In Monday's incident, authorities said O'Keefe used his cell phone to try to capture video of Flanagan and Basel, who posed as telephone repairmen -- wearing fluorescent vests, tool belts and hard hats, one equipped with a hidden camera -- and asked to see the phones at Landrieu's office. Dai is accused of waiting outside in a car with a listening device that could pick up transmissions.
