The two fires in April may be related to other blazes at the old campus in January and in December, Waltham officials said in a statement.

There have been four fires in that area since December," said Waltham police Chief Kevin O’Connell. "These incidents are dangerous, and we’re deeply concerned that someone is going to be hurt.”

The Massachusetts Department of Fire Services and the Waltham Police and Fire Departments are investigating two suspicious fires on the Fernald School grounds last month, and are offering a reward for information.