As recent world events highlight, terrorism risk appears to bechanging. Small groups and "lone wolf" terrorists are now a growingconcern.

Companies and insurance clients may be questioning whether theyare adequately insured for business interruption and other propertylosses.

Terrorism risk insurance

Prior to Sept. 11, 2001, when the World Trade Center and thePentagon were attacked by terrorists, insurers provided terrorismcoverage to their commercial insurance customers essentially freeof charge because the chance of property damage from terrorist actswas considered remote, according to the Insurance Information Institute. AfterSeptember 11, insurers began to reassess the risk.

Related: Terrorism risk insurance: Trends, take-up &pricing

Concerned about the limited availability of terrorism coveragein high-risk areas and its impact on the economy, Congress passedthe Terrorism Risk Insurance Act. The act provides a temporaryprogram that, in the event of major terrorist attack, allows theinsurance industry and federal government to share losses accordingto a specific formula.

The Terrorism Risk Insurance Act was signed into law onNov. 26, 2002, and renewed again for six years in January2015. The new law is known as the Terrorism Risk Insurance Program ReauthorizationAct of 2015.

Major property losses

Unfortunately, terrorism has been around for decades. Remember whenIRA car bombs and hijacked airplanes where always in the news?

Here are the 20 most costly terrorist acts by insured propertylosses:

In this March 17, 1992, file photo, firemen and rescueworkers walk through the debris of Israel's Embassy after aterrorist attack in Buenos Aires, Argentina. The bombing in BuenosAires was widely blamed on Tehran. (Photo: Don Rypka/APPhoto)

20. Bomb attack on Israel's embassy in Buenos Aires.

March 17, 1992

Buenos Aires, Argentina

Insured property losses: $51 million.

Fatalities: 24.

(Photo: iStock)

19. Hijacked Ethiopian Airlines Boeing 767-260 ditched atsea.

Nov. 23, 1996

Indian Ocean

Insured property losses: $60 million.

Fatalities: 127.

The wreckage of a double-decker bus with its top blown offand damaged cars scattered on the road at Tavistock Square incentral London in this July 7, 2005, file photo. (Photo: SangTan/AP Photo)

18. Four bombs explode during rush hour in a tube andbus.

July 7, 2005

London

Insured property losses: $63 million.

Fatalities: 52.

Merchants return to their burned out businesses in thePettha area of downtown Colombo, Sri Lanka on Aug. 1, 1983, to seewhat can be salvaged following a week of rioting. More than 1,000businesses and homes of Tamil's were destroyed in the racial riots.(Photo: JLR/AP Photo)

17. Riot.

July 25, 1983

Sri Lanka

Insured property losses: $63 million.

Fatalities: 0.

In this December 1988 file photo, a police officer walkspast the damage in Lockerbie, Scotland, caused by Pan Am Flight 103from London to New York. The Dec. 21, 1988, explosion would quicklytransform the Scottish town into a byword for international terror.(AP Photo)

16. Bomb explodes on board of a Pan Am Boeing 747.

Dec. 21, 1988

Lockerbie, Scotland

Insured property losses: $76 million.

Fatalities: 270.

The wreckage of the Barajas International Airport car parkin Madrid, Jan. 2, 2007. Rescue workers keep searching for twopeople missing in the rubble of a thunderous car bomb blast blamedon the Basque separatist group ETA that shattered a nine-month-oldcease-fire the group had described as permanent, officials said.(Photo: Daniel Ochoa de Olza/AP Photo)

15. Bomb exploded in car garage at Barajas Airport.

Dec. 30, 2006

Madrid, Spain

Insured property losses: $78 million.

Fatalities: 2.

(AP-Photo/Fotopres)

14. Bomb attack on a newly built, still unoccupied prison.

March 27, 1993

Weiterstadt, Germany

Insured property losses: $95 million.

Fatalities: 0.

In this Nov. 29, 2008, file photo, an Indian soldier takescover as the Taj Mahal hotel burns during gun battle between Indianmilitary and militants inside the hotel in Mumbai, India. (Photo:David Guttenfelder/AP Photo)

13. Attack on two hotels; Jewish center.

Nov. 26, 2008
Mumbai, India

Insured property losses: $113 million.

Fatalities: 172.

London's financial district. (Photo: iStock)

12. Bomb explodes in financial district.

April 11, 1992

London

Insured property losses: $128 million.

Fatalities: 0.

The tail of the hijacked Pan American 747 jumbo jet is theonly part left after the huge plane was blown up by Palestiniancommandos, Sept. 7, 1970. The jet was one of three hijacked overEurope to the Middle East that day. (AP Photo)

11. Hijacked PanAm B-747 dynamited on ground.

Sept. 6, 1970

Cairo, Egypt

Insured property losses: $148 million.

Fatalities: 0.

Smoke billows from hijacked BOAC VC-10 and Swissairjetliners Saturday, Sept. 12, 1970, after guerrillas set off blastsat desert airstrip at Al Khana, Jordan. The TWA Boeing 707 iscompletely concealed by smoke from fires the explosion set. (APPhoto)

10. Hijacked Swissair DC-8, TWA Boeing 707, BOAC VC-10dynamited on ground.

Sept. 12, 1970

Dawson's Field (unused RAF airstrip in desert), Zerqa,Jordan

Insured property losses: $170 million.

Fatalities: 0.

Thousands of search and rescue crews attend a memorialservice in front of the Alfred P. Murrah Federal Building inOklahoma City on Friday, May 5, 1995. (Photo: Bill Waugh/APPhoto)

9. Truck bomb exploded in front of governmentbuilding.

April 19, 1995

Oklahoma City, Oklahoma

Insured property losses: $195 million.

Fatalities: 168.

Recovery operations were in progress at the Haulbowlingnaval base near Cork, Ireland, June 23, 1985, following the AirIndia Boeing 747 crash off the Irish coast which killed all 329people on board. Irish sailors, accompanied by a nurse, carry oneof the bodies recovered from the sea. (Photo: John Redman/APPhoto)

8. Bomb explodes on board of an Air India Boeing 747.

June 23, 1985
Irish Sea, North Atlantic

Insured property losses: $217 million.

Fatalities: 329.

This Saturday, Feb. 10, 1996, file photo shows thedestruction of a bomb blast at South Quay in London's Docklandsarea in which 30 people were injured. The bomb exploded nearBritain's tallest high-rise, Canary Wharf Tower, just an hour afterthe IRA announced it was aborting a one and a half year-oldcease-fire. (Photo: Alastair Grant/AP Photo)

7. Irish Republican Army (IRA) bomb explodes in South KeyDocklands.

Feb. 9, 1996

London

Insured property losses: $347 million.

Fatalities: 2.

(Photo: iStock)

6. Rebels destroy 3 airliners, 8 military aircraft andheavily damage 3 civilian aircraft.

July 24, 2001

Colombo, Sri Lanka

Insured property losses: $533 million.

Fatalities: 20.

A gaping hole caused by an explosion in an undergroundgarage at New York City's World Trade Center reveals various levelsof parking beneath the twin towers complex, March 8,1993. (Photo: Richard Drew/AP Photo)

5. Bomb explodes in garage of World Trade Center.

Feb. 26, 1993

New York City

Insured property losses: $837 million.

Fatalities: 6.

(Photo: iStock)

4. Bomb explodes in financial district.

April 10, 1992

London

Insured property losses: $899 million.

Fatalities: 3.

(Photo: iStock)

3. IRA car bomb explodes near shopping mall.

June 15, 1996

Manchester, England

Insured property losses: $996 million.

Fatalities: 3.

(Photo: iStock)

2. Bomb explodes near NatWest tower in the financialdistrict.

April 24, 1993

London

Insured property losses: $1.22 billion.

Fatalities: 1.

Cars are buried under debris in the aftermath of the Sept.11, 2001, attacks. (Photo: Shawn Baldwin/AP Photo)

1. Hijacked airliners crash into World Trade Center andPentagon.

Sept. 11, 2001

New York City; Washington, D.C.; Pennsylvania

Insured property losses: $25.15 billion.

Fatalities: 2,982.

(Source of data: Swiss Re. Includes bodily injury andaviation hull losses. Updated to 2015 dollars by the Insurance Information Institute using theU.S. Bureau of Labor Statistics CPI Inflation Calculator.)

Related: 6 tips for helping employees get through aterrorist attack