NEW YORK (CNN/Money) - Fired Arthur Andersen LLP partner David Duncan finally took the stand Monday, five days into the Andersen criminal trial, and admitted that he intended to obstruct justice when he directed employees to comply with the firm's document retention policy.

Duncan, the government's star witness against Andersen, said he knew that Enron Corp. documents would be destroyed.

"Yes, I obstructed justice," Duncan said. "I instructed people on the [Enron audit] team to follow the document retention policy, which I knew would result in the destruction of documents."

Duncan's statements were the first time the fired audit partner has publicly spoken about his role in the shredding of Enron documents. During congressional hearings earlier this year, Duncan invoked his Fifth Amendment right against self-incrimination and declined to testify.

Duncan said Monday that he wasn't guaranteed leniency in exchange for testifying against his former firm. The former audit partner agreed in April to plead guilty to one count of obstruction of justice and admitted overseeing the destruction of documents in an attempt to thwart the government's investigation of Enron.

Duncan will continue testifying Tuesday.

Under his deal with the government, Duncan is immune from further prosecution related to the Enron case as long as he continues to fully cooperate with federal authorities -- which could include testimony at future trials -- and agrees not to sell his story or otherwise profit from the debacle.

But he could still face prison and Judge Melinda Harmon, who is overseeing the criminal trial, has warned Duncan that his sentence could be more severe than anything prosecutors might have discussed. He faces a maximum penalty of 10 years in prison and a $250,000 fine and remains free until his sentencing hearing on Aug. 26.

Andersen fired Duncan in January after he directed the destruction of Enron documents.

Some of Monday's testimony focused on Duncan's rapid rise at Andersen. The fired partner revealed that he earned $700,000 last year.

Federal Prosecutor Andrew Weissman also questioned Duncan as to the close ties between the companies. Andersen served as Enron's auditor for 16 years before it was fired in January.

Enron executive Sherron Watkins, the whistleblower that wrote an August letter warning former CEO Kenneth Lay about accounting problems at the energy company, is a former Andersen employee. Jeff McMahon, who told congressional investigators that he was transferred from his position as treasurer after complaining about conflicts posed by Enron's partnerships, is also from Andersen. McMahon resigned in April as Enron's president.

Former Enron Treasurer Ben Glisan is also a former Andersen employee. The Wall Street Journal has reported that Glisan has approached prosecutors about possibly cooperating in the case in exchange for immunity or a possible plea bargain.

"It went very well because [Duncan] told the truth," attorney Barry Flynn, who represents Duncan, said Monday of the former executive's testimony.

Duncan has said that the October e-mail from Andersen in-house counsel Nancy Temple, reminding employees as to the firm's document retention policy, spurred the destruction of Enron documents.

Chicago-based Andersen stands accused of destroying "tons" of Enron documents while it knew of a pending federal investigation. Duncan is expected to make the government's case stronger against Andersen by implicating others at the firm, legal experts have said.

Federal prosecutors also filed a motion Monday to stop a mysterious witness from invoking his or her constitutional right against self-incrimination by letter. The government is trying to force "Witness X" to assert the Fifth Amendment in front of Judge Melinda Harmon, who is overseeing the criminal trial against Andersen.

Last week, prosecutors forced Kate Agnew, a former member of Enron Corp.'s auditing team, to appear in court and take the Fifth in front of Harmon.

But another government witness, Andersen lawyer Temple, was allowed to remain silent and assert her Fifth Amendment right against self-incrimination in a letter Friday.

Temple had been expected to remain silent. In congressional hearings in January, she denied that she told anyone to destroy documents. But in March, when quizzed by attorneys in a class-action lawsuit against Andersen, she refused to testify when asked about her role in the shredding.

As a result of her taking the Fifth, Temple is not expected to appear in the Houston criminal trial.

Rusty Hardin, Andersen lead defense attorney, complained last week that the government was trying to intimidate and embarrass witnesses.

The Andersen criminal trial resumed Monday with the testimony of Amy Ripepi, an Andersen partner who heads the firm's professional standards group in the United States and was called as a government witness.

Ripepi testified Monday that she didn't think that Temple's October e-mail was designed to cause staffers to start shredding documents. Another Andersen partner, James Green, also said he didn't think Temple's reminder was meant to cause employees to begin destroying documents.

But Ripepi did say that Andersen executives knew of problems with the Enron audit. Her testimony will likely add to evidence brought forth last week by prosecutors that shows Andersen had a motive to destroy documents. The Chicago-based accounting firm was already on probation with federal regulators for its involvement in the Waste Management Inc. accounting scandal.

Andersen was the auditor of Waste Management when, in 1998, the company had to restate its earnings. Waste Management had inflated profit by $1.4 billion and Andersen signed off on that company's financial statements. Andersen last summer agreed to pay the Securities and Exchange Commission $7 million to settle charges related to Waste Management, the largest civil penalty ever against a Big 5 accounting firm.

As part of the fine, Andersen agreed to an injunction that forbade it from future wrongdoing.

Ripepi, under cross-examination last week, also provided another possible reason as to why Duncan had overseen shredding of Enron documents. She testified Friday that in a conference call with Duncan and other Andersen partners on Oct. 20, three days before the government says the shredding began, she suggested that severe problems with the Enron audit might lead to disciplinary action against Duncan by the American Institute of Certified Public Accountants.

While accountants are licensed state-by-state, the New York-based AICPA does have some enforcement power and evaluates how accountants comply with accounting rules and the audits they perform.

Under AICPA rules, a bad audit -- or one that does not comply with Generally Accepted Auditing Standards -- can trigger an investigation by an industry panel. The committee can force a firm to fire the lead partner on the audit or take the partner off of all audits, according to Susan Coffey, vice president of self-regulation at the AICPA.

Under questioning by Hardin, Ripepi testified on Friday that during the conference call she mentioned the panel and what it might mean for Duncan.

That testimony offers another reason as to why Duncan directed the shredding, according to Hardin. Evidence that Duncan was trying to avoid an accounting industry probe is being used by the attorney to show that Duncan was trying to save himself by directing the shredding. For Andersen to be found guilty of obstructing justice, the government must show that Duncan shredded the documents to protect the firm.

Houston-based Enron, once the nation's seventh-biggest company, used thousands of off-the-book partnerships to hide nearly $1 billion in debt and inflate profits, according to a report by its independent directors. The company filed the largest bankruptcy in United States history on Dec. 2.

Andersen, which had been involved in previous accounting scandals and was on probation with federal regulators, signed off on Enron's financial statements.