In what is being called the first inside heist of its kind, thieves on an air force base in the center of the country reportedly made off with several F16 engines over the last several days, apparently helped by someone working at the site.

Hebrew news website Walla, which based its report on sources in the air force, said an investigation had been opened into the theft.

Military police suspect that the thieves had inside help from a civilian or a soldier serving on the base, since they found no signs of forced entry, meaning the thieves were able to come and go through the base’s gate.

Officials do not have a suspect and are not sure what the engines may be used for, or if the thieves were in fact just after metal. An army source called the incident “grave” and said it was the first of its kind.

In June of last year, military police opened an investigation after reports surfaced that several F15 and F16 jet engines had been stolen from the Tel Nof air base, near Rehovot. Military officials at the time would only confirm that airplane parts had been stolen.

Arms theft has been an issue for the army over the last several months. In May, two all-terrain vehicles were stolen from the Ketziot army base in the south of the country, and in January, some 1,500 tank shells were snatched from another base in the south.