Houston ? A federal judge in Houston has frozen millions of dollars in assets controlled by Andrew S. Fastow and his family, sources said Friday, in the latest sign that prosecutors are closing in on Enron Corp.’s former chief financial officer.

The court order, signed Thursday night by U.S. Magistrate Judge Calvin Botley, remained under seal. Earlier in the week, the government signaled it would attempt to seize nearly $20 million from Fastow and his relatives after a key Fastow aide, Michael J. Kopper, pleaded guilty to conspiracy and detailed Fastow’s alleged role in financial schemes related to Enron’s collapse.

Sources familiar with the matter said the government moved quickly to freeze Fastow’s assets after his brother, Peter Fastow, tried to move millions of dollars from one of the targeted assets.

A spokesman for the Fastow family, Gordon Andrew, declined comment Friday. Peter Fastow did not return telephone calls. Bryan Sierra, a Justice Department spokesman, also declined comment.

Fastow is a central figure in the investigation of Enron’s collapse into bankruptcy last December. He designed several of the Houston energy trader’s off-balance-sheet partnerships, which concealed the company’s mounting debts and funneled millions of dollars to relatives and friends, according to court papers. Fastow, a protege of former Enron chief executive Jeffrey K. Skilling, left the company last fall after his role in the controversial partnerships was revealed to Enron’s board of directors.

Among the assets the government has targeted are bank and brokerage accounts in the names of Fastow, his relatives and a charitable foundation they control. The list includes $500,000 from Peter Fastow’s account at stockbroker Charles Schwab Inc., as well as Fastow’s Houston home. Prosecutors also indicated they may go after $3 million in assets belonging to former Enron employees who profited from investments in one of the company’s controversial business partnerships, including the Houston home of former Enron in-house lawyer Kristina Mordaunt and the 2000 Lexus sport utility vehicle registered in the name of her husband.

Legal experts said the prosecutors’ move to freeze Fastow’s assets was not unusual given the information they had obtained about apparent efforts to remove funds from an account.

“The prudent thing to do is to go ahead and freeze them,” said Dan Hedges, a criminal defense lawyer and former U.S. attorney for the Southern District of Texas. “In money laundering and drug cases, the assets are frequently frozen even before people are indicted.”

The government still must win formal approval to seize the money a move that could be contested by Fastow and his family members. Legal experts said that prosecutors must prove the funds are the tainted proceeds of criminal activity.