In a letter obtained by the Daily Herald, Dugan said, "my main motivation in pleading guilty was to take responsibility for these crimes, express my remorse and for the truth to finally emerge."
Two other men, Rolando Cruz and Alejandro Hernandez, were wrongly convicted of Nicarico's murder. Both served time on death row before they were eventually cleared.
Dugan is already serving two life terms for two other murders.
Next month a jury is scheduled to decide if he should be sentenced to death for the Nicarico murder.