BOSTON -- Two men accused in a terror plot to kill Americans face new federal charges that they conspired to provide personnel, advice, assistance and other services to Al Qaeda.

A grand jury on Thursday added charges of conspiring to provide material support to a designated foreign terrorist organization in a superseding indictment against 27-year-old Tarek Mehanna of Sudbury, Mass., and 28-year-old Ahmad Abousamra. Authorities say Abousamra, who used to live in Mansfield, Mass., is now in Syria.

The men face up to 15 years in prison and a fine of $250,000 if convicted of the latest charge.

When reached by phone, Mehanna's lawyer, J.W. Carney Jr., did not comment.

The suspects already face a 10-count indictment. Prosecutors say the men conspired to kill American troops in Iraq, assassinate two unnamed U.S. politicians and shoot shoppers in U.S. malls.