A plastic bottle containing a suspicious liquid found over the weekend on the Yurikamome Line in Tokyo was the latest in a series of such incidents, police said Monday.
Three similar discoveries have been reported at subway and train stations in Tokyo and Chiba Prefecture over the past month.
The Metropolitan Police Department and Chiba Prefectural Police suspect one person is responsible for all the incidents and are trying to determine both the motive and the composition of the liquid.
Letters saying "I placed the liquid (in the stations and trains)," were sent to several media organizations and police are investigating their connection to the incidents.
Police said a plastic bag containing a strong-smelling liquid was discovered at 6:15 p.m. June 10 in a men's restroom at Kasai Station on the Tokyo Metro's Tozai Line.
A janitor found the bag on the floor of the restroom and immediately reported it to the police.
The liquid had a strong odor similar to chlorine, police said. No one was injured.
In Sunday's incident on the Yurikamome Line, a plastic bottle containing a suspicious liquid was found after passengers who got off at Ariaketenisu-no-mori Station complained of a strange odor on the train. Staff of the Tokyo Waterfront New Transit System found a plastic bottle containing a liquid in the second car.
Police said the liquid appeared to be hydrochloric acid.
Two female passengers complained of sore throats after the incident.
A plastic bag containing a suspicious liquid was found at Myoden Station on the Tozai Line on June 17, and another on a train at JR Chiba Station.