Officers are searching a garden area near the city, Julia Meyer, a spokeswoman for the state prosecutor in Braunschweig, told CNN on Tuesday.
McCann went missing in 2007 while on holiday in Portugal with her family. She was aged three at the time of her disappearance.
The long-running mystery of what happened to the missing toddler has captivated people around the world.
In June, prosecutors in Braunschweig told CNN they had evidence that a man jailed for an unrelated crime had killed McCann — but not enough to charge the suspect in court.