Explosions were heard late Monday in a public park near the U.S. Army's Camp Zama in Kanagawa Prefecture in what may have been a mortar attack carried out by subversives, police said.

Investigators said Tuesday that one of at least two projectiles believed to have been fired at the military installation hit a nearby house but that no one was injured.

Police found two metal pipes about 55 cm long and 5.5 cm in diameter in the west part of the park and believe they were used to launch the projectiles. The pipes were pointed at the camp, which is about 300 meters northwest of the park.

Investigators also found a battery and burned patches of ground about a meter away from the launchers. The battery was apparently used to run what appears to be a timing device.

Police said witnesses reported hearing several explosions in the area and suspect at least two projectiles were fired.

According to the local fire department, residents in the neighborhood of Zamayatoyama Park, in the Iriya district, made emergency calls shortly after 11 p.m., saying they saw a streak of fire after hearing explosions.

Police said two men, believed to be in their 30s, were spotted running from the vicinity of the park toward Zama Station on Odakyu Electric Railway's Odawara Line.

Camp Zama, which occupies 234 hectares of land and stretches over the cities of Zama and Sagamihara, functions as a key logistics center for the U.S. forces in Japan.

In February, a similar explosion was reported near a high-school baseball field in Yokohama about 400 meters south of the U.S. Navy's Koshiba Fuel Terminal.