Kidnappers released 15 school children on Friday whom they had abducted from a bus while on their way to school near Nigeria’s oil-rich and restive southern delta, a brazen attack highlighting the growing insecurity in the West African nation, a police spokesman said.

Federal police spokesman Emmanuel Ojukwu told The Associated Press that authorities recovered the children early Friday morning, four days after gunmen seized them from a bus travelling in Nigeria’s Abia state. Mr. Ojukwu said the nursery and primary school students were in good condition and soon would be reunited with their parents.

Mr. Ojukwu said he did not have any details about what led to the children’s release, nor did he know if a ransom had been paid. Typically, most kidnap victims are released in a week or two after their families pay whatever ransom they can scrape together.

Authorities said the gunmen stopped the school bus on Monday morning as it headed toward the Abayi International School. The gunmen seized all the mobile telephones from the students, the bus driver and a teacher onboard before taking the children away, officials said. The kidnappers apparently demanded more than $130,000 to release the children, but later lowered the ransom amount.

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