The British government says one U.K. missionary who was kidnapped in Nigeria last month has been killed but three others have been freed.

The Foreign Office said Monday that Alanna Carson, David Donovan and Shirley Donovan have returned to their families, but Ian Squire "was tragically killed."

The four were abducted in the Niger Delta region on Oct. 13.

In a statement, their families thanked Nigerian authorities for negotiating their release.

It was unclear when and how they were freed or how Squire died. The Foreign Office said it could not comment during an ongoing investigation.

Kidnappings for ransom are common in Nigeria. A number of high-profile abductions have been carried out by militants in the Niger Delta region and by the Islamist group Boko Haram in northeastern Nigeria.