The Department of Corrections said it couldn't determine how Helms was hurt and asked the State Bureau of Investigation to look into the accusations.
"After discussing the case with agents from the SBI and reviewing the report, which included witness statements, medical records and video footage, I have determined that there is insufficient evidence to prosecute any crime in the matter," Alexander County District Attorney Sarah Kirkman said.
Helms was sent to prison for three life terms after a fatal drunken driving accident in 1994. Last August, guards say he started a fire in his cell and they had to go in and get him out.
Helms claimed that the guards beat him with clubs, but video of the incident shows guards - not carrying clubs - go in and pull the unresponsive Helms from his smoky cell.
Helms' lawyer - Lynne M. Holtkamp - said the case may now end up in civil court.
"The beating of a person is unlawful, regardless of where it occurs and who it is and who is doing it. Those who are responsible can and should be held accountable," Holtkamp said.