Yet the workers' comp issue, he adds, is almost a bigger issue than property.

"In certain parts of the United States you have very large concentrations of people," Tucker says. "Unfortunately if you were to have another terrorism event, the risk could be physical and include human damage."

And there isn't a backstop like TRIPRA to support the workers' comp market in the event of a major attack affecting a large number of employees. From a commercial underwriting perspective, the fate of TRIPRA—extended or non-extended—could potentially impact the way XL underwrites certain types of risks, he says.

"A couple of clients have come to XL already and said, 'We're in a situation where potentially TRIA could change. What are solutions that XL could offer?'" says Tucker.

Carolyn Snow, secretary of the board of directors for the Risk and Insurance Management Society Inc. (RIMS), notes that workers' comp could suffer if TRIPRA were not renewed or extended: "I think there would be a real reluctance from the part of insurance companies to provide that cover if they didn't have the backstop."

Rob Cruz, regional executive, southwest for Hiscox USA, shares Snow's assessment. He serves as the carrier's product line expert for war, terrorism and political violence. "We do property and some liability, but the impact on the TRIA renewal has a huge impact on the workers' comp line of business, D&O and all the other lines covered under the law."

"What we usually get are requests for property damage and business income," he continues. The majority of claims made after the Boston Marathon's dual bombing were BI loss claims. "I call that 'the Boston Event' because it still hasn't been classified as a terrorism event by the government."

Risk managers should know the parameters of TRIPRA and be prepared for "multiple scenarios" of ways in which claims could be impacted, Cruz says. To trigger TRIPRA, the event in question must be classified as an act of terrorism by the U.S. secretary of the treasury, the secretary of state and the attorney general.

Additionally, as TRIA has been extended and modified over the past decade, insurers have become responsible for covering a greater share of terrorism losses. Today, to trigger TRIPRA backstop funds, there must be proof that inflicted damages from the violent act reach a minimum of at least $100 million (up from $5 million in the original act, and $50 million in the extension act).

The Massachusetts Division of Insurance noted in early September that P&C insurers have paid about $1.18 million in insurance losses since the April 15 bombing in Boston.

In addition to workers' comp, captive insurers could see the biggest changes if TRIPRA is allowed to run its course, XL's Tucker says. "These large multinationals and large U.S. corporations have a single parent captive that has access to TRIA. They would be really left in a very difficult position. We're starting to think about alternative solutions to help those types of clients."

For the most part, however, the market for terrorism claims is largely overseas, Tucker says. And while the breadth of potential terrorism insurance clients in the United States is "considerable," including everything from large multi-national corporations to smaller companies to private individuals, the likelihood of a large number of U.S. companies filing claims against a terrorism policy has historically been lesser than other countries.

As a terrorism marketplace, most of the terrorism claims in recent years have come from mainly outside the U.S., Tucker says. He refers to publicized events, such as the November 2008 Mumbai shooting and bombings that left more than 100 people dead, as well as terror events in Pakistan, Israel, Libya, and Syria, plus "a number of foiled events in last few years."

Arthur J. Gallagher (UK) Limited in London has seen six Terrorism Risk claims submitted from countries involved in the Arab spring and continued uprisings in Bahrain and Egypt, as well as similar disturbances in Colombia, Mexico and Pakistan, says Pamela Fox, associate director for Terrorism and Political Violence.

Many Americans fear further terrorist attacks on U.S. soil when it's actually more common for an attack to occur overseas, he says.

"The frequency of events outside the U.S. is actually much higher," Tucker says. "But the number of companies that actually buy cover outside the U.S. is lower, which is counterintuitive."