Washington ? The missiles that were fired Thursday at an airplane filled with Israeli tourists as it left Kenya’s resort city of Mombasa are readily available on the international black market, relatively easy to use and probably in the hands of Osama bin Laden’s al-Qaida network, the leading suspect in the attack.

U.S. officials have worried for some time that terrorists might turn such heat-seeking shoulder-fired missiles against commercial jetliners.

“You need to be mindful of and concerned about the fact that these things are fairly small, and it is not difficult to smuggle them anywhere,” said a U.S. government official, speaking on condition of anonymity.

The SA-7 Strela, the Soviet-designed shoulder-fired projectile that was fired at the Israeli airliner, is the missile that most worries counterterrorism experts. Two Strela launchers were found after the attack.

In May the FBI warned American law enforcement agencies and airlines that intelligence reports indicated that Islamic extremists may have smuggled SA-7s and U.S.-made Stinger anti-aircraft missiles into the United States. The reports were based on monitors of Internet chat rooms that Islamic extremists use.

The SA-7 is a missile with a high explosive warhead and an infrared guidance system that homes in on engine heat. The United States and other countries make similar missiles.

The SA-7 has an effective range of more than two miles and can hit aircraft flying as high as 13,500 feet.

Thousands of SA-7s, known to Western militaries by its NATO acronym of Grail, were exported from the former Soviet Union after the Cold War, and many are believed to have ended up in the hands of terrorists.

Commercial airliners make large targets as they pass low near airports. And they aren’t equipped with the anti-missile defense systems that are on many military planes. Such systems are designed to detect incoming missiles and divert them by spewing clouds of flares and metal strips known as chaff.

In a measure of the mounting concern over airliners’ vulnerability to missile attacks, the state-run Israel Armament Development Authority earlier this year unveiled a commercial version of an anti-missile system it builds for military helicopters.

It was not immediately known if the Boeing 757-300 owned by Arkia airlines that escaped destruction Thursday was equipped with the system, called Britening.

Eric Doten, a private aviation-safety consultant and former Federal Aviation Administration official, said U.S. airlines hadn’t installed such systems on their planes because the threat was considered to be primarily overseas.

Moreover, he said, portable anti-aircraft missiles aren’t an especially reliable means of downing large passenger aircraft. Most big planes have more than one engine and could still fly after losing one to a heat-seeking missile, he explained.

“There is a good possibility you are going to be able to bring the aircraft around and land it” even if it’s hit, Doten said.

An anti-missile system costs about $2.98 million per plane to install, which also may be one reason that the cash-strapped U.S. airline industry has not rushed to equip jets with them. Doten said U.S. airlines might begin to show “greater interest” in anti-missile systems as a result of the Mombasa attack.

The former Taliban militia of Afghanistan had SA-7s, and Israeli authorities have found them in intercepted weapons shipments bound for the Palestinian territories. Hezbollah guerrillas based in Lebanon reportedly have tried to hit Israeli planes with SA-7s, and Saudi authorities in June arrested an alleged al-Qaida member suspected of trying to down a U.S. aircraft with an SA-7 as it took off from a base in Saudi Arabia.

The SA-7 was first produced for the Soviet military in 1968, and it also was supplied to pro-Soviet regimes. The North Vietnamese army used them to shoot down American planes during the Vietnam war. China, Pakistan and Egypt produce their own versions of the SA-7.