Nigerian authorities say four American hostages were released Wednesday after three weeks in captivity.
Gunmen captured the four contractors on May 9 in the country's oil-producing Niger Delta region. The men are employed by the U.S. oil company Chevron, which operates in the area.
Earlier this week, kidnappers released a Polish oil worker who was abducted near the southern Nigerian city of Warri.
Nearly 20 other foreigners remain in captivity in the Niger Delta.
Some kidnappings have been carried out by criminal gangs, while others are the work of militants who want impoverished local villages to get more of the region's oil wealth.
More than 100 foreign oil workers have been kidnapped in the Niger Delta this year. Most have been released unharmed, often after their employers paid a ransom.
Nigeria's oil exports are down nearly 25 percent since last February because of the unrest.
Some information for this report was provided by AP and Reuters.