Thieves stole £19,000 from an East Ham man during an attack at Kings Cross station.
The victim, 31, changed the money into euros at a bureau de change at Victoria station on Friday, July 31.
He took the tube to Kings Cross where three men attacked him, taking the bag which contained the cash.
He and another man chased the attackers into a nearby hotel, where they threatened a member of the public before fleeing with the money.
British Transport Police have released images of four men they believe have important information and would like to speak to about the incident.
If you know them, or have any other information, call 0800 405040, or text 61016, quoting reference B4/TSUB of 1/8/2014. You can also call Crimestoppers anonymously on 0800 555111.
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