It also found no evidence there were additional conspirators in the bombing - the worst terrorist attack on American soil which left 500 people injured.

Some 500 were injured in the blast

The county grand jury, which determines whether there is enough evidence to bring charges, did return one sealed indictment, believed to concern witness tampering, but no details were available.

Two men have been jailed on federal charges of conspiring to blow up the Alfred P Murrah federal building on 19 April, 1995.

Gulf War veteran Timothy McVeigh, 30, was sentenced to death while his accomplice Terry Nichols, 43, was jailed for life. Both are appealing against their convictions.

The county grand jury also called on District Attorney Bob Macy to carry through with plans to bring state murder charges against the men.

Conspiracy theories

Despite the convictions, some people have continued to speculate there was a wider conspiracy, including the presence of a third man referred to as John Doe No 2.

The 21-page jury report said conflicting testimonies from 26 witnesses made a final determination impossible.

''We cannot affirmatively state that absolutely no one else was involved in the bombing,'' the panel said. ''However, we have not been presented with or uncovered information sufficient to indict any additional conspirators.''

The grand jury was convened in July 1997 to investigate whether other people were involved and whether federal authorities had advance knowledge of the bombing but failed to prevent it.

McVeigh has been sentenced to death

But it determined that the Federal Bureau of Alcohol, Tobacco and Firearms did not receive prior warning of the blast.

And it ruled out a theory that the bombing resulted from a government sting operation that went awry.

The panel also found that there was a "single bomb" and not two, as some reports said.

The 15 jurors came to their conclusions after evaluating testimonies from 117 witnesses and examining more than 1,900 documents over 18 months.