They were seized on 4 April from their office near the south Darfur state capital Nyala.

BBC: Foreign aid staff freed in Darfur

Nyala, where the early Sunday morning attack occurred, is about 300 kilometers (185 miles) south of Kutum.

CNN: STORY HIGHLIGHTS

Dr. JOHN-KALL: I'm based in a place called Nyala in South Darfur.

NPR: Doctor Describes Scene in Darfur

Late last month fighting broke out in Kalma, a vast camp for internally displaced people near the town of Nyala in south Darfur.

ECONOMIST: Trouble in Sudan's Darfur region

Nyala is more of a city than you'd imagine.

NPR: Doctor Describes Scene in Darfur