Andersen is accused of obstructing justice by shredding documents and deleting computer files related to audits of Houston-based Enron Corp.
They told U.S. District Judge Melinda Harmon that the only reason the government subpoenaed four company partners and employees already interviewed by federal agents was that prosecutors wanted to strengthen their case against Andersen.
Such uses of grand juries have been banned by courts.
"It is only after obtaining the indictment — and realizing that instead of getting a plea of guilty it might actually have to prove its case at trial — that the government sought to call any fact witnesses," as opposed to government agents acting as summary witnesses, before the grand jury, the motion said.
At a hearing last week, government attorney Samuel Buell argued that the government is allowed to continue a grand jury inquiry that could result in additional charges or defendants.
The judge has set a May 6 trial date for the charges against Andersen.