SALEM, Mass. (AP) — A fourth person has pleaded guilty to his role a scheme to sell illegal MBTA passes.

The Massachusetts attorney general’s office said 28-year-old Alex Saunders of Beverly pleaded guilty Friday in Essex Superior Court to charges of receiving stolen property and conspiracy and was sentenced to 18 months in jail. Prosecutors said he helped sell some of the thousands of passes in the biggest fare evasion scheme in the transit agency’s history.

Three others pleaded guilty and were sentenced Thursday, including Andres Townes, who made more than 20,000 unauthorized passes worth about $4 million when he worked at a Beverly subcontractor that makes the official passes. Two women also pleaded guilty to helping sell them.

Authorities said about half the passes were never circulated. The scheme was discovered last year by an alert train conductor.

Copyright 2012 The Associated Press.