An apparently accidental explosion injured three U.S. Marines on Thursday, two days after gunmen opened fire on Marines in a terrorist attack elsewhere in Kuwait.

None of the injuries was believed life threatening, said Lt. Chris Davis at the U.S. Navy's Fifth Fleet headquarters in Bahrain.

"It seems to be an accident and we are investigating," Davis said.

In Washington, Pentagon officials said the explosion, perhaps from a land mine, occurred as Marines were setting up a training exercise on the Udari range on the Kuwait mainland.

The Marines' injuries included puncture wounds in the arms and legs, the officials said.

U.S. troops have trained in Kuwait for more than a decade without reported hostile encounters until Tuesday, when two gunmen in a pickup truck opened fire on Marines engaged in urban assault training on Failaka, an island 10 miles east of Kuwait City. One Marine was killed and a second injured. After driving to a second location and attacking a second time, both Kuwaiti shooters were killed by Marines.

U.S. and Kuwaiti officials labeled Tuesday's attack terrorism, and White House spokesman Ari Fleischer said links to Osama bin Laden's al-Qaida terrorist network could not be ruled out. Several Kuwaitis have been tied to bin Laden. Most notably, al-Qaida spokesman Sulaiman Abu Ghaith, who was stripped of his Kuwaiti citizenship in October 2001, and Kuwaiti-born Khalid Shaikh Mohammed, who is suspected of being a Sept. 11 mastermind.

The gunmen in the Tuesday attack -- Anas al-Kandari, 21, and his 26-year-old cousin, Jassem al-Hajiri -- had trained in Afghanistan and were angry about Israeli killings of Palestinians, a friend and a relative said.

Al-Kandari's brother, Abdullah, said Abu Ghaith was a distant cousin and the attackers did not know him.

Officials in Kuwait said they were questioning about 30 people to determine the attackers' motives and learn more about them. Sheik Mohammed Al Sabah, minister of state for foreign affairs, said people "we think provided assistance to the terrorists" were being rounded up.

Kuwait's independent Al-Qabas daily reported Thursday that Kuwaiti authorities were holding two medical students linked to al-Kandari. Quoting a "senior security source," it said the medical students, who are brothers, had plans and documents that indicated they were preparing to attack an unidentified "large multistory" target.

Mohammed al-Awadi, a cleric who knew Al-Kandari, said the gunman had "chosen to walk in the footsteps of Osama bin Laden."

Al-Kandari spent 18 months in Afghanistan, and his cousin al-Hajiri joined him there for six months, al-Awadi said. Both returned days before the Sept. 11, 2001, terror attacks in the United States that are blamed on bin Laden's al-Qaida organization, the cleric said.

In a second incident, troops driving from Camp Doha to a site nearer the Iraqi border Wednesday said a man drove along side and pointed a gun at them. One of the Marines shot at the vehicle which veered off the road. Officials later said occupants of the civilian vehicle claimed only to have been holding a cell phone. No one was injured in the incident.

After a U.S.-led coalition liberated Kuwait in the 1991 Gulf War, the small oil-rich state signed a defense agreement with the United States under which the American military carries out exercises.

The latest war games, dubbed Eager Mace, started Oct. 1 and involve 1,000 Marines from the 11th Marine Expeditionary unit based in Camp Pendleton, Calif., and 900 sailors on the amphibious transport ships USS Denver and USS Mount Vernon.

The Pentagon said Eager Mace 2002 was routine and not related to any possible war to oust Iraqi President Saddam Hussein.