MASON COUNTY - The last of the co-defendants in the murder case of Michigan State Police trooper Paul Butterfield was convicted on May 6 after accepting a plea agreement. Tammi-Lynne Spofford, 50, of Irons, plead no contest in Mason County's 51st Circuit Court to accessory after the fact of the murder, which was committed by her son Eric Knysz on Sept. 9, 2013. "This has been a highly emotionally charged case from the outset, which has directly affected many persons in Mason, Manistee and Lake counties," said Paul Spaniola, Mason County Prosecutor. "I'm not certain whether closure can ever occur in this particular case, but today's events will go a long way toward the healing that needs to happen." Spofford was charged in connection to the case in November 2013. In February, she rejected a plea deal that would have required her to testify against her son. As part of Spofford's new plea agreement, a charge of unlawfully driving away a motor vehicle was dismissed. The agreement also provides that she will be sentenced within the Michigan Sentencing Guidelines applicable to the case, and that her incarceration may not be greater than one year. Her sentencing is set for 3:30 p.m. on June 17. Eric Knysz, then 19, of Irons, shot and killed Butterfield during a routine traffic stop on Custer Road in Mason County. Butterfield conducted the stop at 6:20 p.m. and a passing motorist called 911 three minutes later to report finding the trooper on the ground with a bullet wound to the head. Knysz and his wife Sarah then drove to Spofford's house in Lake County, according to testimony from Knysz's trial. Eric, Sarah and Spofford drove to a home in Mason County where Eric stole a vehicle, and Spofford assisted by driving the original vehicle away from the scene. A "be on the lookout" was issued on the stolen vehicle and officers from multiple agencies caught up to Eric and Sarah in Dublin. Butterfield died that night at the Munson Medical Center in Traverse City. Knysz was convicted of the murder on Feb. 25 and sentenced to life in prison without parole on April 8. On April 14, just days after being admitted to the Michigan Department of Corrections' Charles Egeler Reception & Guidance Center in Jackson, he attempted suicide by hanging himself in his cell. Following the suicide attempt, he was placed on life support for three days and pronounced dead on April 17. Sarah Knysz, 21, of Irons, is serving two to five years in prison for accessory after the fact of the murder and unlawful driving away of an automobile. Spofford is free on bond until sentencing.