After pleading guilty to motor vehicle homicide Monday morning, retired Harvard Law School Professor Detlev F. Vagts ’48 was sentenced to three years of probation and six months of house arrest.

Vagts, who spent 40 years as a professor at the Law School, retired in 2005. He started teaching at the Law School in 1959 and currently holds the title of “Bemis Professor of International Law, Emeritus.”

Last summer, Vagts struck and killed Marcia Kearney, 54, a Newton Country Day school teacher.

Vagts, 82, was driving partially in the opposite lane of travel when he hit Kearney at the intersection of Grant Avenue and Ward Street.

Kearney was making a left onto Grant Avenue when the collision threw her from her Vespa.

Following the crash, Vagts drove 448 feet before returning to the location of the accident. Kearney was located approximately 82 feet from the collision, according to a press release from the District Attorney’s Office.

Vagts and his wife were lost and “running late for a dentist appointment” near Hammond Street, according to police reports.

Newton District Court Judge Dyanne J. Klein sentenced Vagts to six months of house arrest in addition to three years of probation. He will be confined to his home with a GPS monitoring bracelet and will only be allowed to leave for his or his wife’s doctor’s appointments.

Klein ordered Vagts not to drive for the rest of his life, exceeding the 15-year mandatory loss of license associated with vehicular homocide.

Vagts could have faced up to two-and-a-half years in jail.

“This defendant has now admitted and accepted responsibility for his role in the death of Marcia Kearney,” Middlesex District Attorney Gerard T. Leone Jr. ’85 said in a press release.

“This is a tragic incident where negligent operation of a motor vehicle resulted in the death of an innocent woman. We commend the court for ensuring that this defendant does not drive ever again. Our thoughts remain with the victim’s family and friends,” he said.

Klein also ordered Vagts to write a letter of apology to Kearney’s family and pay restitution for storage fees, which will be determined at a Dec. 12 hearing.

—Staff writer Julia L. Ryan can be reached at jryan@college.harvard.edu.