A gunman shot and killed three oil workers

A gunman Tuesday shot and killed three oil workers -- an American, a Canadian and a Yemeni -- and then killed himself, according to the U.S. Embassy.

All worked for Hunt Oil in Marib.

Hunt Oil said the shooting took place on the company's oil rig.

In December, a suspected Islamic militant killed three U.S. humanitarian workers and wounded a fourth at a missionary hospital in Yemen. Shortly after the attacks, the suspect told authorities he did it "to get closer to God."

Security in Yemen has been a U.S. priority since the October 2000 attack on the destroyer USS Cole in the port of Aden that killed 17 sailors. The attack was blamed on terrorist group al Qaeda, which was responsible for the September 11, 2001, attacks in New York and Washington.

Yemen, the ancestral homeland of al Qaeda leader Osama bin Laden, is a hotbed for extremist Islamic militant groups that find refuge in the country's mountainous, tribal strongholds.

In November, a CIA-operated Predator drone fired a missile that killed bin Laden's top lieutenant in Yemen, Qaed Salim Sinan al-Harethi, and five other al Qaeda suspects.

It is thought that a number of al Qaeda leaders may have fled to Yemen after being pushed out of Afghanistan, but the United States has warned of threats to Americans from a number of Islamic militant groups in the country.