TOKYO — Two homemade mortar launchers found within possible striking distance of Naval Air Facility Atsugi on Wednesday are being investigated as a planned guerrilla attack, Japanese police said.

The time-triggered devices contained projectiles and were found at 10:20 a.m. in a wooded area about 500 yards north of the main base gate in Ayase City by a Japanese neighbor, a Kanagawa Prefectural Police spokesman said Wednesday.

No projectiles had been launched from the devices, the spokesman said.

Further details on the launchers and whether the projectiles were explosive were unavailable Wednesday, according to the spokesman.

Police suspect an attack was planned by an extreme leftist group, the spokesman said.

Base officials will provide any requested support, but Kanagawa police are in charge of the investigation, said Atsugi base spokesman Tim McGough.

In the meantime, the base has raised its security level and is informing residents about the incident, McGough said.

“We’re making sure residents stay aware of anything suspicious and report it as soon as they see it,” McGough said.

The incident is the second such publicly announced device found near a mainland Japan military base in the past two months.

At Yokota Air Base, about an hour north of Tokyo and two hours north of Atsugi, police found two 4-foot-long metal pipes with wires attached in November. Police found no signs of use at the time.

In September 2008, a mortar blasted a hole into the balcony of an off-base home near Yokosuka Naval Base, south of Tokyo.

A group referring to itself in Japanese as “revolutionary army” claimed responsibility for the attack and called it a protest of the aircraft carrier USS George Washington’s arrival in Japan.