SAN`A, Yemen (AP) _ The United States said Wednesday it was sending a team of investigators to help determine why a Yemeni man opened fire on oil workers, killing an American, a Canadian and a Yemeni before committing suicide.

The Yemeni Interior Ministry said the motive in Tuesday's attack was believed to be personal. The assailant, identified as Naji Abdullah al-Kumaim, had worked for the oil rig's owner, Nabors Drilling Co., for seven years and suffered from depression, it said.

Al-Kumaim was not known to have a political affiliation, and he shouted during the shooting that was taking revenge against people who were filing reports about him, the ministry said.

The attack occurred at an oil field in the northern province of Marib, about 100 miles northeast of the Yemeni capital San`a.

The U.S. Embassy identified the American victim as Ron Horsch. His age and hometown were not immediately available. However, Hunt Oil Co. said in a statement that the American was a superintendent employed by the Dallas, Texas-based company.

``Motives behind the incident are not yet clear, so it's premature to speculate on why this happened,'' U.S. State Department spokesman Richard Boucher said in Washington.

The embassy said the United States was sending a team to help with the investigation.

A statement issued Tuesday by Hunt Oil Co., which supervises the oil rig, said the four men worked at a rig owned and operated by Nabors, and were shot while working in the field.

Hunt Oil said the two Canadians and the Yemeni victim, in addition to the killer, were Nabors employees.

The Yemeni victim was identified as Nazem al-Kabati. The names of the Canadians were not released by the Yemeni authorities or the Canadian Embassy in Riyadh, which handles Canadian interests in Yemen.

However, a Foreign Affairs Department spokesman in Ottawa identified the dead man as William Sivell, 45, of Buena Vista, Saskatchewan.

The wounded man was identified by Canada's CBC radio as Mark Edwards, 38, of Edmonton, Alberta. He suffered abdominal wounds and was being flown to Geneva, Switzerland on Wednesday, said Yves Duval, Canadian Embassy spokesman in Riyadh.

Hunt Oil Co. said it had suspended drilling operations in Yemen ``for the foreseeable future.''

Gun violence is not unusual in the largely lawless tribal areas of Yemen, but the country also has been the scene of several Islamic militant terror attacks on Western targets in recent years. The country is the ancestral homeland of al-Qaida leader Osama bin Laden.

In 2000, an explosives-laden boat rammed an American destroyer, the USS Cole, off Yemen, killing 17 U.S. sailors. A suicide attack last October killed a Bulgarian crew member on a French oil tanker and spilled 90,000 barrels of oil into the Gulf of Aden.

On Dec. 30, 2002, a suspected Islamic militant shot dead three U.S. Christian missionaries who worked at a Baptist-run hospital.