Lagos, Nigeria (CNN) -- The Nigerian army has arrested a militant gang leader believed to be behind the kidnapping of 19 people, authorities said Saturday.
Fifty-one of his followers were also arrested, said Lt Col Timothy Antigha, a spokesman for the military taskforce in the Niger Delta.
Nigerian authorities on Wednesday freed the 19 hostages seized in the oil-rich Niger Delta region.
They were 12 Nigerians, two Americans, two Frenchmen, one Canadian and two Indonesians.
Nigeria has launched an offensive targeting Delta, Bayelsa and Rivers states, the largest military campaign in the Niger Delta region since the government offered an amnesty program in 2007.
The Movement for the Emancipation of the Niger Delta is one of several militant groups in the region that has been battling the government for years over the distribution of the country's oil wealth.
CNN's Christian Purefoy contributed to this report.