Portland police are warning about a string of catalytic converter thefts in the city.

There have been 12 thefts and attempted thefts of catalytic converters between Feb. 11 and April 11, according to the Portland Police Department.

That comes as police across the state say that catalytic converter thefts remain a serious problem.

Thieves target catalytic converters for the precious metals inside, which can fetch, depending on the quantity, between $150 and $1,500 a piece, according to the Maine State Police. For that reason, the sale of catalytic converters is tightly controlled. Sellers are required to present photo identification and sign an affidavit swearing the parts aren’t stolen.

The majority of the Portland thefts have happened in dimly lit parking lots with little security surveillance. Many of the vehicles have been left unattended for days, and thieves are targeting box and commercial trucks, vans, and hybrid vehicles, according to the police department.

The thefts remain under investigation.