The 200-pound chimpanzee went berserk in February after Herold asked Charla Nash to help lure him back into her house. The animal ripped off Nash's hands, nose, lips and eyelids.
Cohen said that there was no record of the animal attacking anyone previously, and that it had interacted with Nash many times before the attack.
The decision not to file charges "does not in any way minimize the horror that we all feel with what occurred and with the horrendous injuries suffered by Ms. Nash," Cohen said. "Our prayers go out to her and her family."
Messages were left for attorneys for Herold and Nash, who revealed her heavily disfigured face last month on "The Oprah Winfrey Show."
Nash's family is suing Herold for $50 million and wants to sue the state for $150 million. Nash's family has said Herold was negligent and reckless for lacking the ability to control "a wild animal with violent propensities."
A biologist for the state Department of Environmental Protection warned officials before the attack that Travis could seriously hurt someone if he felt threatened, noting that he was large and strong.
But Cohen said Monday there's no evidence those concerns were conveyed to Herold.
Herold's attorney has called the attack work-related and said her family's case should be treated like a workers' compensation claim. The strategy, if successful, would limit potential damages in the case and insulate the chimp owner from personal liability.
Test results showed that Travis had the anti-anxiety drug Xanax in his system. Cohen said it is impossible to say what effect, if any, the drug had on the animal.
The chimp, which was shot and killed by police, had also escaped in 2003 from his owner's car and led police on a chase for hours in downtown Stamford. No one was injured.
Records obtained by The Associated Press through an open-records request show the state began receiving warnings immediately after that event.
Nash's attorney has said the environmental department had information for at least five years that would have allowed the agency to remove Travis from the home.
Environmental protection officials have said that during the 13 years Travis was with Herold, the agency received only a few inquiries about the chimp among thousands in general about possession of wild animals.
They said the memo from the biologist underscored the need for a clear, new law that would forbid ownership of potentially dangerous animals as pets and impose stiff penalties for those possessing them, and they blamed the failure to act on a communications problem and a lack of expertise in exotic animals at the agency.
