In this Friday, Jan. 27, 2017 photo, an Iraqi officer holds a drone belonging to Islamic State militants in Mosul, Iraq. Islamic State is hacking store-bought drone technology, using rigorous testing and tactics that mimic those used by U.S. unmanned aircraft to adapt to diminishing numbers of fighters and a battlefield that is increasingly difficult to navigate on the ground. (AP Photo/Khalid Mohammed)
In this Friday, Jan. 27, 2017 photo, an Iraqi officer holds a drone belonging to Islamic State militants in Mosul, Iraq. Islamic State is hacking store-bought drone technology, using rigorous testing and tactics that mimic those used by U.S. unmanned aircraft to adapt to diminishing numbers of fighters and a battlefield that is increasingly difficult to navigate on the ground. (AP Photo/Khalid Mohammed) Credit: Khalid Mohammed

Mosul, Iraq — Faced with a diminishing number of fighters, the Islamic State group is relying on retrofitted commercial drones to guide suicide car bombers to their targets and to launch small-scale airstrikes on Iraqi forces.

The extremist group is spending freely on drone technology as it faces pressure from coalition forces, hacking store-bought machines, applying rigorous testing protocols and mimicking tactics used by U.S. unmanned aircraft.

In all, a half-dozen storehouses IS used to make and modify drones have been found recently in Mosul, Iraqi military officials said.

The Associated Press this week visited the largest drone workshop uncovered so far, a warehouse in the Shura neighborhood. Scattered among stacks of paper were pieces of Styrofoam wings, fins and radio transmitters piled in the corners of the factory.

Most of the completed drones were destroyed by IS fighters as they retreated, Iraqi officers at the warehouse said. Spreadsheets the fighters left behind showed purchases totaling thousands of dollars a month for drone equipment.

One receipt dated a few months before the operation to reclaim Mosul began recorded the purchase of wires, silicon, electrical plugs, cables, rotors and GoPro cameras. Handwritten notes instructed IS drone operators to write daily “mission reports” and monthly reports “about the challenges and difficulties you face as well.”

All the accounts were headed “board of development and military manufacturing,” some sub-headed “air observation division.”

A cache of documents also obtained this month in a smaller makeshift factory by a researcher in Mosul indicates that the group is testing small drones — normally used as playthings — with deadly intent.

The researcher, Vera Mironova, said the drone paperwork she discovered signals a program for having machines make up for a shortage in manpower. The documents included part lists in English and Arabic. One file, marked “Tool Kit,” contained a checklist of several dozen essentials: GoPro and chargers; battery cable; laptop; explosives; and devices made up Items 1-5.

Mironova said the use of drones to both drop explosives and to direct more deadly payloads was an adaptation to the decrease in the number of attackers available.