HOUSTON -- Federal authorities appeared to take control of Enron's headquarters here Tuesday in response to mounting accusations that vital documents were being shredded by the bankrupt energy empire's employees.

Enron's reflective glass skyscraper at 1400 Smith St. in downtown Houston was ringed by security guards as FBI agents worked in the floors above to secure the site of the alleged shredding.

No visitors were allowed in the building, and employees were sometimes seen carting out items.

Enron said the FBI's arrival was not a raid. Rather, it said, the company had requested the investigation after renewed allegations this week that its employees were shredding documents.

"We can confirm the Department of Justice and FBI personnel are on site conducting an investigation of the allegation raised," said a company spokesman.

Meanwhile, in Washington, congressional investigators said they would subpoena top executives at Andersen, Enron's former auditors, forcing them to appear at hearings set for Thursday.

Rep. Billy Tauzin, R-La., the chairman of the House Energy and Commerce Committee, was to sign four subpoenas Tuesday night, said committee spokesman Ken Johnson. The subpoenas are for Andersen CEO Joseph Berardino, attorney Nancy Temple, senior executive Michael Odom and David Duncan, the lead auditor on Enron who was fired last week for directing the destruction of documents.

"Mr. Berardino found the time to make his case last weekend on "Meet The Press," Johnson said. "We believe it's important for him to make the same case before Congress as well."

Duncan's attorney, Robert Giuffra, had asked to delay his client's testimony. He said Tuesday evening that he had received no subpoena. Andersen also said that no subpoenas had been received as of that time.

"We have directly told the committee that we are willing to testify," said Andersen spokesman Patrick Dorton. "It's only a question of when."

Tauzin's committee is one of at least 10 congressional inquiries into the nation's largest bankruptcy. Thursday's hearing is to focus on Andersen's destruction of Enron documents.

On Wednesday, the Senate Finance Committee, which oversees tax law, intends to send a letter to Enron, asking for public release of the company's tax returns and other tax information. On Friday, the committee plans a meeting with the Internal Revenue Service to discuss Enron's use of tax shelters.

Back at Enron headquarters in Houston, FBI agents were dispatched to secure the building even as a hearing proceeded elsewhere in the city before U.S. District Judge Melinda Harmon.

That hearing was over a lawsuit by Enron investors and originally was about document shredding not by the company but by Andersen.

On Tuesday, the attorney representing investors, William Lerach, brought a box of shredded paper to the federal courthouse, saying that was the evidence Enron had provided to him.

Adding substance to his allegation, this week a former Enron manager, Maureen Castaneda, said the company was shredding documents as recently as last week. She made the disclosure in an interview with ABC News.

Castaneda was an Enron official in charge of assessing risks in international business, such as foreign exchange fluctuations and unstable governments.

The allegations, if true, would clash with Enron's previous claims that it ordered employees to preserve all documents last October, when it learned that the Securities and Exchange Commission was investigating the company's accounting practices.

Andersen acknowledged last week that its employees shredded thousands of documents as an investigation by the SEC kicked into high gear. Andersen, which earned $52 million a year in fees from Enron, was fired by the company last week amid widespread criticism that it turned a blind eye toward shady practices in an effort to protect a big account. The accounting firm has denied it did anything improper, saying Enron's problems are the result of a failed business model.

Roughly $62 billion in market value has been wiped out since Enron shares, which are now a penny stock, peaked at around $90.