Grant also testified that there were indentations on the letters from where someone had written on another envelope that had been on top of them in a stack.
The indentations were analyzed under a light source and turned out to be for Curtis' former addresses in Booneville and Tupelo, though one of the addresses was spelled wrong, Grant said.
McCoy said the evidence linking the 45-year-old to the crime has hinged on his writings posted online, which were accessible to anyone.
So far, Curtis is the primary focus for investigators and the only person arrested in connection with sending the letters, but Grant testified Friday that authorities were still trying to determine whether there were any co-conspirators.
Family and acquaintances have described Curtis as a caring father and enthusiastic musician who impersonated Elvis and others but struggled for years with mental illness.
His writings show he was consumed by trying to publicize claims of a conspiracy to sell body parts on the black market.
Curtis' adult daughter, Madison Curtis, and brother Jack Curtis, both said after Friday's hearing that they'd never heard him criticize President Obama, though he was vocal in his feelings about many politicians.
His ex-wife, Laura Curtis, has said she does not believe he sent the letters.
Associated Press writer Jay Reeves contributed to this report.
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