TOKYO -- The hunt for the attackers who ravaged Tokyo's subway system with nerve gas led police Tuesday to a hospital bed -- where reports said a man suspected of planting a poisonous parcel was himself recovering from the attack's effects.


Japanese television and newspapers said police were guarding the man while waiting for him to recover sufficiently for questioning in Monday's attack, which according to latest figures killed eight people and sickened thousands more.


Police said 4,708 people had been treated and nearly 700 remained hospitalized. Another 75 people remained in critical condition, officials said.


The three subway lines contaminated by sarin, a highly toxic nerve gas, resumed full operation Tuesday after military chemical-warfare experts sprayed chemicals in subway cars and platforms to neutralize the gas.


Sarin, a nerve gas developed by the Nazis in World War II, is heavier than air, so it is difficult to remove from underground subway tunnels.


Although subway officials insisted the trains were safe, the ridership was about 30 percent less than normally seen on a public holiday; the stock market, government offices and many businesses were closed for the Spring Equinox.


All trash cans were removed from 148 stations on the three lines because of fears of a follow-up attack. Some other train lines also removed or covered over trash containers, but there were no reports of unusual objects being found.


In a bizarre development, newspapers reported that trading in the stock of a company that has a monopoly on gas-mask manufacturing in Japan was 100 times above average Thursday and Friday, the two trading days before the nerve gas attack.


A spokesman for the company, Shigematsu Works, called the surge in turnover "visibly unnatural" and was at a loss to explain it.


Bookstores, meanwhile, reported strong sales of a 1991 British novel, translated into Japanese, with an eerily similar plot to Monday's attack. In the book, "Deadly Perfume," researchers study the impact of a scheme to spread anthrax, a powerful bacteria, in subway systems.


There were no credible claims of responsibility for the subway attack. However, police said they had received about 30 eyewitness descriptions of suspicious persons.


News reports said the hospitalized suspect had been seen by several people placing a plastic bag wrapped in newspaper on the floor by the door of a train.


When he got off, a passenger who was suspicious of the man kicked the object onto the platform. It began to emit white fumes and the suspect collapsed, the reports said. Two people died at that station.


Police officials would say only that they were investigating the reports.


A cult-like religious group named Aum Shinri Kyo again denied Tuesday it was involved in the attack. The group has been linked in news reports to several mysterious releases of irritating gases and unsolved kidnappings.


Three members were arrested Sunday on suspicion of kidnapping a student who wanted to leave the group.