A 25-year-old woman has died after being struck by a freight train near Santon Downham.
She has been identified as Tracey Henman, who lived on a barge on the River Little Ouse.
A 45-year-old man has also been arrested on suspicion of false imprisonment as police try to establish the events that led to her death.
The woman died on Monday night at around 9.30pm near a level crossing between Santon Downham and Brandon.
Prior to that, police had received a call concerning a domestic incident on a boat on the Little Ouse in the Brandon area.
At 9.30pm, while making active inquiries into the earlier call, Suffolk Police received a call from the driver of the train, saying his freight train had just hit someone.
Suffolk Police have launched a joint investigation with the British Transport Police into the incident.
Officers are now looking for a woman who was reported to be on the riverside and the boat, in Brandon, on Monday.
Detective Inspector Andy Footer said: “The boat was tied up at moorings near the Brandon Ram on Monday afternoon.
“At some point during the afternoon the woman got on to the vessel, spoke to the occupants and took a ride along the river in the boat.
“We need to trace this lady as she may be able to assist our investigation. She is described as in her late 50s or early 60s with grey/blond hair, and was wearing a lilac jacket. She had a purple mountain bike with her at the moorings.”
Police are also looking for anyone who may have seen a woman in distress in the forest around Santon Downham before 9.30pm on Monday night.
Due to earlier police involvement, the matter has been referred to the Independent Police Complaints Commission as a matter of course.