German police believe he may have information about the disappearance 17 years ago.
McCann disappeared while on holiday in 2007.
The main witness in the case could be prosecuted for "unsworn false statements".
It comes 15 years after Madeleine's parents were put under suspicion for their daughter's disappearance.
The large operation saw sniffer dogs, radars and search teams used.
"You can use it to pick any lock, including security locks."
A coalition of officers from Portugal, Germany and the United Kingdom conducted the three-day search.
Police officers were collecting unspecified samples by a reservoir in Portugal.
But Braunschweig's state prosecutor is remaining tight-lipped about what the tip-off was.