A US federal court jury has cleared a Russian company of violating copyright law by selling a software program that cracked digital locks used to secure electronic books.

Moscow-based ElcomSoft was charged with violating the controversial 1998 US Digital Millennium Copyright Act (DMCA).

The case caused international uproar after ElcomSoft programmer Dmitry Sklyarov was arrested in July 2001 following a talk he gave about the program at the DefCon hacker conference in Las Vegas.

Sklyarov agreed to provide testimony against his boss in return for the charges against him being dropped.

After two days' deliberation the jury decided that ElcomSoft had not "wilfully" violated the DMCA.

"[ElcomSoft] never intended to violate the law," defence attorney Joseph Burton told Reuters.

ElcomSoft's program, which sold online for about a month in mid-2001 for $99 before Adobe complained, allowed users to make copies of e-books, transfer them to laptops and have the computer read them aloud.

The company's president, Alexander Katalov, testified that he did not think the program, which is legal in Russia, was illegal in the US. He said it was meant only for legally bought e-books.

The US Attorney for Northern California said in a statement: "While disappointed, we are also pleased that the judge upheld the constitutionality of the DMCA ... to bring these cases."

Civil liberties groups welcomed the outcome.

The Electronic Frontier Foundation said that the ruling sent a strong message to federal prosecutors who believe that tool makers should be thrown in jail just because a copyright owner does not like their tools.