Togolese police say they are probing what could be a cross-border network of killers after six beheadings in Togo and one in Benin earlier this month.
One of four suspects has admitted he belongs to a gang wanted in Benin for similar crimes, Togolese police say.
The victims were all attacked over a two-day period and their decapitated bodies were drained of blood.
A BBC correspondent says there are fears these are ritual murders for witchcraft purposes ahead of Christmas.
In some traditional practices, human body parts are used in the belief that they make charms more powerful.
According to Togo's police chief Col Damehane Yark, one of the four suspects in custody has also admitted to killing the youngest victim, a 12-year-old schoolgirl.
The BBC's Edow Godwin in the Togolese capital, Lome, says the killers struck in the southern districts of Vo and Lac, in Maritime province that borders Benin.
At the same time a teenage girl was beheaded in Benin, 65km from the city of Cotonou in similar circumstances.