The threat interrupted a busy Mass and touched off a large police response days after the worst massacre of school-age children in U.S. history.
Halfway through the noon service at the St. Rose of Lima Church, the priest stopped and said, "Please, everybody leave. There is a threat," said worshipper Anna Wood of Oxford, Conn.
At least a dozen police in camouflage SWAT gear and carrying guns soon arrived. An Associated Press photographer saw police leave carrying something in a red tarp. Guns drawn, they searched the church and adjacent buildings.
Deborah Metz, a Trumbull police officer on the scene, gave the all-clear after about an hour. Police said the church would be on lockdown for the rest of the day.
Brian Wallace, spokesman for the diocese, said someone called and "threatened to disrupt the mass."
Gunman Adam Lanza, his mother and eight of the child victims attended St. Rose of Lima. It is a Roman Catholic Church with an adjacent school, which Lanza attended briefly.
The church hosted overflow crowds at all three morning Masses Sunday.
Wood said everyone left calmly but described a congregation on edge. One boy, about 9, left with his mother.
"He asked his mom, 'Mom, why are we leaving?'," Wood recalled. "The mom couldn't answer. She just started crying."
Nancy Elis lived in Newtown for 28 years before moving to nearby Southbury in 2006, but she was back Sunday to visit her former church.
With police searching the property, she said the scene looked like "a war zone."
"This is the house of God," she said, sobbing. "Why would they have to leave for fear for their lives? What's become of our nation?"