Castresana said evidence shows Rosenberg bought two cellular phones, one to communicate with his killers and another to deliver threatening messages to his own personal phone.
He said the commission's theory is that Rosenberg was motivated by a sense of guilt and frustration over what he believed was the government's involvement and failure to properly investigate the killing of his client and girlfriend, Marjorie Musa, along with her father, Khalil Musa.
Rosenberg had advised Khalil Musa to accept a seat on the board of directors of the private- and government-sponsored Rural Development Bank.
In his video, Rosenberg said if he were killed, it would be because he was assassinated by Alvaro Colom, Guatemala's president, to silence him for discovering the killings were linked to money laundering at the bank.
Colom suggested that criminal or political interests were behind the video and expressed relief at the verdict.
"The day has arrived that I have waited for in silence and patience, where this crime finally been clarified," he said.
The commission's investigation has led to 11 arrests. Francisco and Estuardo Valdes Paiz, the cousins implicated in the investigation, are considered fugitives from justice.
Investigators allege they contacted the head of security at their pharmaceutical company to help them find someone to carry out the killing for $40,000.
Rosenberg's accusations were distributed to reporters on DVDs at his funeral and immediately set an already polarised country into a frenzy of protests, allegations of corruption and calls for Colom's resignation.
His posthumous accusations of corruption became a rallying cry for members of Guatemala's dominant elite, many of whom are angry over Colom's attempts to eliminate tax loopholes for corporations and criticise his inability to reduce high rates of violent crime.
Colom is overwhelmingly backed by Guatemala's mostly Mayan Indian poor for his efforts to tax the rich and build schools and clinics for disadvantaged communities.