Knifeman kills four police officers in Paris
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The attacker was shot to death.
Union official Loic Travers said the attack on Thursday appears to have begun in an office and continued elsewhere inside the large compound across the street from Notre Dame Cathedral.
Mr Travers said the motive was unknown, but the employee allegedly responsible for the violence apparently never posed any problems before.
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Hide AdHe said he cannot remember an attack on police officers of this magnitude.
Another union official had said earlier that two officers were injured, one gravely.
French media are reporting the attacker was an employee.
They added that the country's interior minister and the Paris prosecutor were on their way to the scene.