PORTSMOUTH, N.H. — Fire investigators are trying to figure out what caused a fire at the police department shooting range in Portsmouth, N.H.

Fire Chief Steve Achilles says a decision was made recently to recover and study all bullets found at the scene of the July 30 fire and study the remains of rubber backstops in the range.

Achilles tells the Portsmouth Herald there was no immediate sign that tracer rounds were fired inside the range. Those are bullets made pyrotechnic charges added to the base that, when shot, illuminate the paths of fired bullets so shooters can see it and correct their aim.

Deputy Police Chief Corey MacDonald earlier this month said tracer rounds are not authorized for use inside the pistol range, and the department does not buy them.