The 17-year-old suspect led officers to his house in northern Fukushima prefecture (state) and pointed out his headless mother on her futon mattress, with a frank, "It's in here," Kyodo News reported.
He told police he beheaded his mother on the eve of her 47th birthday while she slept, and added, "It didn't matter who I killed," Kyodo reported. Police confirmed the arrest.
Japan has long prided itself on safe streets and low incidents of violent crime. But the latest killing triggered soul searching about what some worry is a disturbing new trend — particularly with its mix of youthful aggression and icy remorselessness.
"If true, it's horrifying," Chief Cabinet Secretary Yasuhisa Shiozaki said.
Last year, a jobless man allegedly cut up his mother and entombed her body parts in concrete-filled buckets left abandoned in a yard.
In January, Tokyo was on put edge on when a woman confessed to dismembering her husband with a saw and dumping the parts around the capital.
Then just weeks ago, the nation was riveted by a high-profile verdict in the death of a British hostess whose body was also dismembered and stashed in a seaside cave.
While dismemberment crimes are not unique to Japan, experts say such cases are on the rise as Japan grapples with the fallout of a decade of economic malaise. Alienation and a trend toward objectifying fellow humans is also fueling the violence.
"There have been a high number of incidents involving dismembered bodies and I certainly think there is a chain reaction going on," said Susumu Oda, an expert in criminal psychology at Japan's Tezukayamagakuin University.
Tuesday's case recalls a 1997 slaying in which a 14-year-old boy beheaded an 11-year-old and left his head at a school gate with a sinister note stuffed in his mouth.
That case spurred widespread debate about tougher laws against juvenile offenders.
But dismemberments made headlines again last year, when a man in western Japan reportedly cut up his mother's body and encased the remains in concrete buckets.
The 37-year-old suspect also roasted some of the parts on an electric grill before throwing them in the garbage, apparently to delay decomposition and hide the odor.
In the January case, a woman confessed to bludgeoning her husband with a wine bottle, sawing the corpse in pieces and dumping his body parts around Tokyo.
The confession ended a murder mystery that began the previous month when a man's torso was found in a garbage bag on a Tokyo street.
The man's legs were later found at a separate location in Tokyo and his head was discovered in a suburban park weeks later. That woman told them that the husband had physically and emotionally abused her, local media reported.
Late last month, a serial rapist was sentenced to life in prison for nine attacks, but was cleared of raping and dismembering a British woman after a six-year trial that detailed one of Japan's most disturbing sex crimes.
The British victim, Lucie Blackman, worked at a hostess bar frequented by the suspected, and her body was discovered in a cave near his house — her head also encased in concrete. While the suspected was convicted on other charges, the court said there was no solid evidence linking him to the Briton's death.
While such gruesome killings have outraged the nation, some experts caution that the dismembering of victims is not always a symptom of deeper social disfunction.
"In many cases, they killed in a desperate situation and carried or hid the body by dismembering it to avoid being caught for homicide," said Yoshikazu Yuuma, expert in criminal psychology at Saitama Institute of Technology outside Tokyo.