Mr Mwangura was also accused of illegal possession of drugs. He denies both charges.
Police want to hold Mr Mwangura for five days, saying he has vital information they want to question him about, the BBC's Joseph Odhiambo reports from Kenya.
The claim that the tanks on board the Ukrainian ship were going to South Sudan has been strongly denied by Kenyan and Ukrainian authorities.
But Western sources, who spoke on condition of anonymity, told the BBC that the tanks on board the ship were bound for the autonomous government of South Sudan, in possible contravention of a peace accord.
The pirates aboard the Ukrainian ship, the Faina, reiterated to the BBC on Thursday that they were seeking a ransom of $20m and said they were in good condition.
The Faina is surrounded by international warships, and the pirates have threatened to retaliate if attacked.
A British report says that Somali piracy has more than doubled this year, with more than 60 ships attacked so far and between $20m and $30m (£16.9m) paid in ransom.
The French announcement was made after a meeting of EU defence ministers at Deauville, in northern France.
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