"Numerous studies estimate there are between 300,000 and 400,000 religious congregations in the United States. Many people think of a house of worship as a safe area where violence and emergencies cannot affect them. However, violence in a house of worship is not a new phenomenon and many facilities are developing and updating security and emergency plans and procedures to ensure the safety and security of their congregations, visitors, staff, and facilities. [...] DHS has published multiple bulletins on this trend and advocates increased public vigilance and awareness given the reality of terrorist-inspired individuals who have carried out, or have attempted to carry out, attacks in the United States. DHS recognizes that terrorist-inspired individuals (Domestic Terrorist) and homegrown violent extremists may be encouraged or inspired to target public events or places."
Department of Homeland Security: http://www.dhs.gov/