Chikyuu no Kodomo

Two people, a man and a woman, are hit and killed by a train. They are of different ages and genders, but they have one strange thing in common—they both died with their heads crushed. Makoto, a high school student who lost his older brother in the incident, meets Seiichi, a boy who also lost his mother. They realize that a certain organization is involved in these mysterious deaths. Perhaps the brother and mother did not commit suicide, but were manipulated into dying...? And what is the "strange creature" involved? A summer of suspense begins as the boys pursue the mystery.

(Source: Kodansha, translated)