FORT MEADE, Md. - The Army intelligence analyst blamed for the biggest leak of secrets in U.S. history boasted that he was changing history in a letter accompanying some of the data he allegedly sent to WikiLeaks, according to a witness in the military's case against Army Pfc. Bradley Manning.

"This is possibly one of the more significant documents of our time, removing the fog of war and revealing the true nature of 21st century asymmetrical warfare. Have a good day," the file read.

The text, along with almost 500,000 classified battlefield reports, was on a data card that investigators found among Manning's belongings, according to digital-crimes investigator David Shaver. The letter represented the most forceful piece of evidence yet introduced in a military hearing to determine whether Manning, 24, should be court- martialed.

In a back-and-forth on the digital case against Manning, the prosecution said evidence showed that he communicated directly with WikiLeaks founder Julian Assange, sent the nearly 500,000 classified battlefield reports to the anti-secrecy website, and bragged about leaking video of a 2007 helicopter attack.

Investigators cited a May 2010 exchange between Manning and a mathematician named Eric Schmiedl. "Are you familiar with WikiLeaks?" Manning allegedly asked.

"Yes, I am," Schmiedl wrote.

"I was the source of the July 12, 2007, video from the Apache Weapons Team which killed the two journalists and injured two kids," Manning wrote, according to the prosecution.

Throughout the hearing, Manning's attorneys countered that others had access to his workplace computers and pressed Shaver into conceding that some files in a batch of 10,000 State Department cables on Manning's computer did not match documents published by WikiLeaks. He said an additional 100,000 cables could not be matched to Manning's user profile. But it was unclear if the cross-examination damaged the prosecution's case in any way.

Manning is accused of illegally leaking secret information that surfaced on WikiLeaks, a breach that rattled U.S. foreign relations. If convicted, he could face court-martial on 22 charges, including aiding the enemy.