Police are linking two incidents in which London Underground staff members were threatened with a knife at different stations.
They occurred on Tuesday (November 22), first at Walthamstow Central station at around 2pm, and again a short while later at Bond Street.
British Transport Police (BTP) say they are working "tirelessly" to capture the man responsible, and have issued CCTV footage of a person they would like to speak to in connection with the incidents.
Investigating officer police constable Noel Delpuech-Deasy said: “These are particularly concerning incidents and we are working tirelessly to trace the man responsible.
“Threatening, attacking or intimidating anyone who uses the railway, including members of staff, is inexcusable and we will not tolerate it.
“I would urge anyone who has any information to contact us as soon as possible.”
Anyone with information can text BTP on 61016, or call 0800 40 50 40 quoting reference 222 of 25/11/2016.
Information can also be passed to Crimestoppers anonymously on 0800 555 111.
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