Portland police are warning residents to remain vigilant while they investigate a series of thefts involving the removal of catalytic converters from motor vehicles.

The thieves, who most often sell the auto parts for cash, are not discriminating about which vehicles they target. Investigators said catalytic converters have been stolen from sedans as well as buses and box trucks that have typically been parked overnight in the city.

“These incidents follow regional and national trends of increasing thefts,” Portland Police Department spokesman David Singer said in a statement.

Portland police are advising vehicle owners to park in well-lit areas whenever possible and to make sure that their vehicle’s security alarms work.

This week, the Sanford Police Department and the city of Sanford issued a similar joint warning about the theft of catalytic converters, which has become an increasing trend regionally and nationally.

Thieves typically can make between $50 and $875 by stealing and selling a converter, depending on the type of precious metals in it. Many lower-emission and hybrid vehicles contain higher amounts of precious metals, and some larger vehicles have multiple catalytic converters, according to Sanford police. The cost to repair a vehicle can be several thousand dollars.

Sanford police made an arrest in connection with the theft of a catalytic converter late last month. Their investigation led to the arrest of a West Newfield man.

The International Association of Auto Theft Investigators advises vehicle owners to consider implementing additional deterrents, such as engraving a VIN or license information into the converter, using automotive exhaust spray paint to mark tag information, or install “ultra destruct labels” that break and leave a mark when removed.

Anyone who witnesses suspicious activity, such as people trying to gain access to the underside of a vehicle, should contact Portland police at 874-8575.