WASHINGTON (CNN) -- About a fifth of Americans think federal agents have listened in on their phone calls, a CNN/USA Today/Gallup Poll released Tuesday suggests.

Twenty-one percent of the 1,000 adults who replied to the survey conducted Thursday through Sunday said it was very likely or somewhat likely their conversations had been wiretapped, while 52 percent said it was not at all likely.

Twenty-four percent said it was not too likely.

The sampling error for the question was plus or minus 3 percentage points.

Shortly after 9/11, President Bush authorized the National Security Agency to conduct electronic surveillance of communications -- phone calls, e-mails and text messages -- between people inside the United States, including Americans, and terrorist suspects overseas, bypassing a secret court set up to provide warrants for such surveillance.

The Bush administration has said the program is designed to monitor terrorists, while critics say the spying is illegal and may infringe on the civil liberties of Americans.

According to the poll, Americans appear to be split over the legality of the domestic eavesdropping program. About 49 percent of respondents said the president had definitely or probably broken the law by authorizing the wiretaps and 47 percent said he probably or definitely had not. (Poll results)

Those numbers were similar to a question about whether the program is right or wrong -- 47 percent said it was right and 50 percent called it wrong.

The sampling error for those questions was plus or minus 5 percentage points.