"These testimonies establish once and for all that we are not alone," said Steven Greer, director of the Disclosure Project, a nonprofit research organization dedicated to disclosing alleged alien sightings.

Greer, who organized the program at the National Press Club in Washington, argued that the United States and other governments have known about UFOs for at least 50 years and have been keeping the information secret.

Greer said there were some 400 witnesses who claim to have firsthand experience with UFO sightings or alien evidence, and are willing to testify before Congress.

Among them is Daniel Sheehan, a well-known Washington lawyer who is acting as counsel for members of Greer's group.

Sheehan told reporters that during the Carter administration he found out about government-held UFO information that then-CIA Director George Bush, father of the current president, would not release.

Sheehan said he was then led into the National Archives where he was shown photographs of captured UFOs, complete with what appeared to be alien writing symbols, but he was only allowed to take notes on a yellow legal pad. He traced the photos onto the cardboard back of his pad, he said.

Military Denies UFOs

The U.S. government repeatedly has denied having any evidence of alien species, though it investigated the possibility for decades.

The Air Force was responsible for investigating alleged sightings for the military. From 1947 to 1969, the service's Project Blue Book at Wright-Patterson Air Force Base, Ohio, investigated 12,618 reported sightings. It said it found explanations for all but 701, such as swamp gas, airplane lights, weather balloons and other natural phenomena.

Sightings for which explanations couldn't be determined were categorized as sketchy reports that couldn't be pinned down.

In 1997, the Air Force announced it was formally closing its nearly 50-year investigation into the alleged alien sighting at Roswell, N.M. It denied that there was evidence of a UFO at Roswell and that the military covered it up.

"Information obtained through exhaustive records searches and interviews indicated the material recovered near Roswell was consistent with a balloon device of the type used in a then-classified project," said a Pentagon statement. "No records indicated, or even hinted at, the recovery of 'alien' bodies or extraterrestrial materials."

Alien Energy?

In another statement Wednesday, Donna Hare, a former NASA contract employee, said that Apollo astronauts saw an alien craft when they landed on the moon, but were told not to reveal it. Hare's source was a man who had been quarantined with the astronauts.

Former Air Force Maj. George Filer III told reporters that when he was at McGuire Air Force Base in New Jersey, an alien craft came down, and an alien got out and was shot by a military policeman.

"Our security police went out there and found him at the end of the runway dead, Filer said.

"They asked me to brief the general staff," he said, but was later told not to.He said he would tell the story in front of Congress.

Greer said extraterrestrials could provide a new, plentiful source of energy that would provide the world's energy needs.

Information from alien encounters, said Greer, could also have significant impact on the global environment and the quest for world peace.

ABCNEWS' Katelynn Raymer in Washington and David Ruppe in New York contributed to this report.