Haicke Petrus Marinus Janssen was born in 1885 in Kampen, Netherlands. He was a bookkeeper in Amsterdam before he became a sailor. In 1913, he was working on the Belgian Red Star Line’s SS Kroonland as a lookout man when she came to the rescue of the Uranium Steamship Company’s SS Volturno after that ship caught fire in the Atlantic Ocean. The United States and United Kingdom awarded him and other seamen involved a medal for rescuing the Volturno’s passengers and crew.
On 12 May 1915 Janssen left Rotterdam for Hull. He arrived there the next day and stayed for a week before travelling south to London and further on to Southampton on the pretext of being a travelling cigar salesman. In Southampton, he stayed in the Crown Hotel from 24 to 28 May and sent five telegrams to the firm Dierks & Co. in The Hague. Not long after, Janssen was arrested by Scotland Yard detectives on suspicion of espionage on behalf of Germany. Dierks & co. was already known to MI5 to be a front company of the German secret service.
The arresting detectives found telegrams from Dierks & Co., a price list containing a secret code, cigar samples and a copy of Jane’s Fighting Ships 1915. During his interrogation at New Scotland Yard, Janssen said he was a jobless seaman. He had been introduced by a friend to a German secret agent as he spoke English and was looking for a job, having failed to find work on a steamer. When Superintendent Basil Thomson asked him whether he knew a man named Willem Roos, he denied. Roos had been arrested on 2 June in London after he wired Dierks & Co. from Edinburgh. He too was a Dutch sailor in the service of the German naval intelligence service N who posed as a cigar salesman.
At the court martial in Westminster Guild Hall on 16 July Janssen admitted the espionage charges brought against him. The court martial sentenced him and Willem Roos to death by firing squad. On the early morning of 30 July 1915, Janssen and Roos were executed in the Tower of London by a firing squad of the Scots Guards. Janssen was first at 6 a.m. He rejected his right to be blindfolded.