WASHINGTON (CNN) -- Government attorneys say members of Congress should not be "given keys to the evidence locker" when investigators search their offices for criminal wrongdoing. But a lawyer defending a Louisiana congressman argues there's a Constitutional "sphere of protection" against interference by the executive branch.

A federal appeals court heard arguments Tuesday on whether the FBI violated what's known as the "speech and debate clause" of the Constitution when agents raided the Capitol Hill offices of Rep. William Jefferson a year ago next week. The clause is intended to block the executive branch's use of law enforcement to intimidate federal lawmakers.

Jefferson, a Democrat, is the subject of a bribery investigation that has been sidetracked after the seized materials ended up in a court battle over access to his office. Federal prosecutors say Jefferson took $90,000 in cash during a sting operation and stashed it in a freezer at his Louisiana home.

Tuesday, government attorney Michael Dreeben told an appeals court the search warrant signed and affirmed last year by a federal judge included "what is necessary to protect legislative independence," such as a "filter team" to segregate any legislative material outside the range of the search.

Jefferson's attorney, Robert Trout, told the three judges on the panel they should void the search warrant and order the materials returned, since the raid "cannot help but intimidate the legislative function" as prohibited by the Constitution.