Mexican authorities are investigating whether human remains found in a park outside the capital belong to 12 young people whose kidnapping in May shocked the city, officials said Thursday.

Mexico City’s top prosecutor, Rodolfo Rios, said the remains of seven people have been recovered since the search began Wednesday and that investigators were looking for more bodies in the rough terrain.

The mass grave was discovered in a park in the town of Tlalmanalco, some 30 kilometers (19 miles) southeast of the capital, and police put up a checkpoint on a dirt road surrounded by corn fields one kilometer from the site.