At 04.00 GMT a break-in was reported at a clothing store, minutes before two men were hit by a train nearby
Police believe two men killed when a quad bike was hit by a train may have been fleeing the scene of a shop burglary.
Two hundred men's specialist outdoor jackets worth £40,000 were stolen from Go Outdoors in Newport Road, Cardiff.
Officers were called to the store just after 0400 GMT - around the time the men, both in their 20s, were hit on tracks in Rumney nearby.
Police are studying CCTV footage and are appealing for witnesses.
In a statement, South Wales Police said: "Police believe that the men who made off from the Go Outdoors store after committing the burglary may be the same men involved in the fatal incident at a nearby railway line.
"Although it is too early to be certain at this stage."
Trevor Davies, 20, was one of the two men who died
British Transport Police said the dead men had been identified as Trevor Davies, 20, and David Cooper, 24, both from the Rumney area of Cardiff.
Detectives are searching for two other people who are thought to have fled the scene after the two quad bikers were killed.
It is believed the train that hit them was travelling at around 70mph.
Two quad bikes were found but it is unclear if the victims were travelling on one or both vehicles.
British Transport Police said the quad bikes had been travelling east towards Newport when they were struck from behind.
The passenger train involved was not carrying members of the public at the time, and had been making its way from the Canton depot in Cardiff to Cheltenham for servicing.
Chief Inspector Sandra England said the two victims were pronounced dead at the scene, close to New Road in Rumney.
She confirmed that officers are also trying to trace two other individuals.
The incident happened on the line near the river bridge and New Road, Rumney
"We now believe two other persons have made off from the accident and we would urge anyone with any information on that to please come forwards so we can trace these people and speak to them," she said.
Ms England said the train driver was "pretty shaken up".
"The investigation is in the early stages at the moment," she added.
"It was dark, in the early hours, when the incident happened and my main plea is that if anybody did see what happened they should call us on 0800 40 50 40.
Rail workers at the trackside following the fatalities
"We will be following up all lines of inquiry to find out what the men were doing there at that time of night."
The crash led to four rail lines being closed for some hours, disrupting morning commuters. Three have since been reopened but continuing delays are likely.
A spokesperson for Network Rail said: "There was some damage to the track in the area of the collision.
"At the moment we are not able to give an accurate update on what repairs are needed or when they will be completed.
"Obviously at the moment, the clear-up operation is very much still under way there."
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