CARSON CITY -- A massive explosion that would have produced a mushroom cloud over the Nevada Test Site was delayed Friday by federal officials.

An attorney representing groups opposed to the test of conventional explosives said he now believes the project is dead.

The Nevada Site Office of the National Nuclear Security Administration announced the delay of the Divine Strake blast following a brief hearing in U.S. District Court in Reno.

Reno attorney Bob Hager, who filed a lawsuit against the project on behalf of several groups and individuals, including the Winnemucca Indian Colony and Nevada and Utah down-winders, said he believes the decision means the project has been abandoned.

"The problem is the way Bechtel and the DOD (U.S. Department of Defense) have conducted themselves since the lawsuit was filed," Hager said. "They used junk science to provide false assurances of the safety of the blast.

"Is it gone for good? I believe it is," he said.

The test, which would detonate a 700-ton bomb containing ammonium nitrate and fuel oil at the test site, 85 miles northwest of Las Vegas, originally was planned for June 2. It then was announced that the test would not go off before June 23.

Critics of the blast expressed concern that radioactive particles left over from above-ground nuclear testing at the site in the 1950s and 1960s could be kicked up into the air from the above-ground blast of conventional explosives.

The announcement of the delay said only that the National Nuclear Security Administration was withdrawing its "finding of no significant impact" connected with the blast.

"This action is being taken to clarify and provide further information regarding background levels of radiation from global fallout in the vicinity of the Divine Strake experiment," it said.

"Atmospheric testing of nuclear weapons by several countries in the 1950s and 1960s resulted in the dispersion of radioactive fallout throughout the northern hemisphere. The efforts of the Nevada Site Office are focused on explaining, in a means clearly understandable to all, what background radiation from this fallout means with respect to the contemplated Divine Strake experiment."

Hager said he believes the U.S. government was not prepared to respond to information he filed Monday, including affidavits from experts from around the country, that said it is impossible to tell from the government information whether the blast would be safe.

Hager noted that a hearing in U.S. District Court scheduled for June 8 was canceled at the request of the U.S. government. But senior U.S. District Judge Lloyd George said he will continue to monitor the project, Hager said.

Steve Erickson, director of the Citizens Education Project and a Salt Lake City plaintiff in the lawsuit, called the announcement good news.

"An indefinite postponement sounds more definite than what we've been told before," he said. "This test can't stand up to scientific scrutiny."

Erickson promised continued vigilance of the proposed blast.

"Our biggest concern is that this is another step down the slippery slope of resuming actual nuclear testing," he said. "This will hopefully curtail that effort by the administration."

Peggy Maze Johnson, executive director of Citizen Alert, said the postponement should be a wake-up call to the government that such tests cannot be undertaken in what she described as the slipshod manner of the proposed Divine Strake blast.

"We can't be doing things in a haphazard way when the health and safety of our citizens is at issue," she said. "I am thrilled beyond words."

Johnson said an event scheduled from 11 a.m. to 2 p.m. Sunday outside the gates of the test site will go on as planned. "But it will definitely be a celebration," she said.

Thomas Wasson, chairman of the Winnemucca Indian Colony and a plaintiff in the lawsuit, said the postponement was good news.

"There was not enough data or information, but the government was going to proceed anyway," he said. "With a test of this magnitude, they should have all the answers for any questions that might arise."

U.S. Sen. Harry Reid, D-Nev., issued a statement saying he was pleased to hear of the delay. All questions need to be answered and all precautions taken before such a test can go forward, he said.

Agency scientists have said the $23 million Divine Strake test is designed to assess how shock waves from existing and future weapons travel through limestone to destroy a tunnel in which weapons of mass destruction are stored.

The proposed blast became national news on March 30 when James Tegnelia, director of the Defense Threat Reduction Agency, told defense reporters about it.