An invitation to a peace conference from a Catholic group prompted a scare at US Representative Peter Welch’s office in Burlington after a worker thought the letter looked suspicious. Authorities locked down the building Monday while a hazardous materials team investigated. The incident followed the weekend arrest of a Mississippi man accused of mailing letters containing the poison ricin to President Obama and US Senator Roger Wicker of Mississippi. Police said the letter to Welch did not contain dangerous material. The Burlington Free Press reported that the envelope was sent from the Fatima Center — Servants of Jesus and Mary, in Constable, N.Y.