From the Background: "Prior to September 11, 2001, commercial P&C insurance policies generally did not exclude coverage for losses resulting from terrorism. The events of September 11, 2001 (September 11 attacks) resulted in approximately $44 billion of P&C insurance losses, more than two-thirds of which were reimbursed by reinsurers to insurers. Following the September 11 attacks, insurers and reinsurers began to exclude coverage for terrorism risk from commercial P&C policies. The Terrorism Risk Insurance Act of 2002 (TRIA) was enacted, in part, because the widespread unavailability of terrorism risk insurance 'could seriously hamper ongoing and planned construction, property acquisition, and other business projects, generate a dramatic increase in rents, and otherwise suppress economic activity.' TRIA established the Program, which requires insurers to make terrorism risk coverage available within certain lines of commercial P&C insurance (TRIP [Terrorism Risk Insurance Program]-eligible lines of insurance)."
U.S. Department of the Treasury: https://home.treasury.gov/