Sixteen-year-old Bruce Grisafe has pleaded guilty to serving as a lookout for the botched robbery that led to the murder of Kyle Sheets aboard a boat in Mystic in September 2008.

Grisafe pleaded guilty to accessory to first-degree robbery, accepting a plea agreement that calls for a prison sentence of up to six years and five years probation. Under the agreement, defense attorney Sebastian O. DeSantis has the right to argue for a shorter prison term when Grisafe is sentenced on May 28.

Grisafe is one of four boys and men charged in connection with the fatal stabbing of Sheets, 29, on a boat that was docked at Seaport Marine in Mystic on Sept. 26, 2008. Police said Christopher Allen and Vincent Green boarded the boat where Sheets lived with his father intending to rob Sheets of money and drugs. Grisafe and Bryan Sutton served as lookouts.  A struggled ensued on the boat, and Allen fatally stabbed sheets.

Allen was sentenced last month to 32 years in prison after pleading guilty to felony murder. Green has also pleaded guilty to felony murder and is scheduled to be sentenced April 19 to 25 years in prison.

Sutton’s case is pending, and his next court date is March 25.