LIBREVILLE, Gabon  — A member of Gabon's senate has been arrested in an investigation into the ritual killing of a 12-year-old girl in the central African nation four years ago, the first time a senior politician has been detained in such a case.

Rising public anger at a spate of ritual killings in Gabon, an oil-rich former French colony on the Gulf of Guinea, sparked a march by thousands of people in the capital Libreville last month after mutilated bodies washed up on beaches.

President Ali Bongo promised the protesters that anyone convicted of such killings would be jailed for life.

Senator Gabriel Eyeghe Ekomie, who was stripped of his parliamentary immunity in December, was arrested on Friday after failing to appear before a court on May 31, his lawyer said.

Eyeghe Ekomie was summoned for questioning by the court after a man convicted of the girl's killing said at his trial in May 2012 that he did it on the senator's orders. Eyeghe Ekomie has denied the accusation.

"This is an unjust decision because my client was not correctly summoned," said lawyer Gisele Eyue Bekale. "We asked the judge to re-issue the summons but he did not. We will continue to appeal this decision."

Human and animal body parts are prized by some in the region, who believe they confer magical powers. Gabon's Association for the Prevention of Ritual Crimes estimates that at least 20 people have been killed so far this year and their lips, tongues, genitals and other organs removed.

Earlier this week, a sack containing human genitalia was found in a building in Libreville. An investigation is underway.

Gabon is not the only African country with a black market trade in human organs.

Grave robbers dug up more than 100 bodies in Benin's capital in November. Cameroonian authorities in September arrested five people for trafficking when they were stopped at a checkpoint with a severed human head.