premise: Gunmen have kidnapped several people from an Italian oil vessel off the Nigerian coast, according to industry and security officials on Wednesday.
  The exact number and the nationalities of the hostages were not immediately known. One security source asked for anonymity said the incident occurred overnight on a vessel belonging to a subsidiary of Italian oil giant Eni.
  Other sources said about 25 foreign workers and 60 Nigerian employees were taken overnight on the vessel, which has not been confirmed officially.
  The vessel was about 50 kms off the coast of Nigeria's Rivers State at the time, officials said.
  The kidnappings were the latest in a series of attack on oil installations in the volatile Niger Delta, where most of the country's oil is produced.
   What does the incident that occurred overnight refer to?
hypothesis: employees were taken overnight on the vessel