By STEVEN R. WEISMAN and JUAN FORERO

WASHINGTON, May 27 - The Justice Department on Friday rejected Venezuela's request for the arrest of a Cuban exile wanted for an airplane bombing as a preliminary to his extradition, saying it had not provided proper supporting evidence.

A State Department official said the Venezuelans were told that their request, which called for the arrest of Luis Posada Carriles to prevent his escape as a first step to extradition, did not contain sufficient information regarding the facts and circumstances of his involvement in the 1976 bombing. The midair explosion of a Cuban airliner off the coast of Barbados killed 73 people, including several Venezuelans.

"The provisional arrest request as submitted by the government was clearly inadequate," the official said. The ruling does not preclude a formal extradition request.

Mr. Posada is in American custody. He escaped a Venezuelan jail in 1985 while awaiting trial on charges he planned the bombing. Now 77, he reappeared on May 17 in Miami.

The Venezuelan government, which said on Sunday that it would consider severing diplomatic ties with Washington if the extradition was denied, responded with a statement, from its embassy in Washington saying it would "present all the necessary documentation to request the extradition."

Reached by phone in his office, the ambassador, Bernardo Álvarez, explained that the "preventive arrest" request had been filed on May 13, before Mr. Posada reappeared, when Venezuela had indications that he had slipped into the United States. The Venezuelan Supreme Court, he said, has called on the government of President Hugo Chávez to make a formal extradition request.

"We are going to request the extradition very soon, presenting all the documentation needed," Mr. Álvarez said.

Mr. Posada, a former C.I.A. operative has placed the Bush administration in a political and legal predicament that has raised questions about whether it has a double standard on terrorism.

Extraditing Mr. Posada would embitter members of Miami's Cuban-American community, some of whom are close allies and financial backers of President Bush and his brother, Gov. Jeb Bush of Florida. Not extraditing him could raise accusations that the administration was willing to protect a bombing suspect if his mission was to eliminate one of Washington's adversaries.

American officials have not said whether they would extradite Mr. Posada, saying that the question was a legal matter. But they have indicated they would not willingly send him to Venezuela. The fear is that Mr. Posada would wind up in Cuba, where he would be executed for a range of crimes, including an assassination attempt on Mr. Castro.

Steven R. Weisman reported from Washington for this article and Juan Forero from Bogotá, Colombia.