Although the overall risk of a terrorist attack on U.S. soil has decreased slightly, the risk of an extreme event occurring within the next five years has grown, according to a new report from Risk Management Solutions.

RMS on Thursday released its fourth annual U.S. Terrorism Risk Model. Although the analysis showed that anti-terrorism measures have reduced the risk of an attack on U.S. soil, the risk of terrorist activity continues to increase globally and the risk of extreme events over the next five years, including terrorist attacks using chemical, biological, radiological, and nuclear weapons, has increased.

"The improvements in U.S. security are making it harder to carry out big attacks, but as we've seen in the attacks in London and Madrid, the terrorists are now using smaller-scale attacks on soft targets to cause civilian casualties," said Dr. Andrew Coburn, director of terrorism research at RMS. "The threat of a mass-casualty attack by a determined group remains a real possibility."