Johnson & Johnson offered a $100,000 reward today for information leading to the conviction of the person who laced Tylenol capsules with cyanide, and authorities said that traces of the deadly chemical found in two bottles from stores two blocks apart came from the same source.

By late today, Food and Drug Administration technicians had sifted through more than 200,000 capsules taken from area stores in a widening investigation. The probe, similar to a 1982 tampering case in Chicago, began when a young woman was found dead of cyanide poisoning last weekend.

"This is an act of terrorism, pure and simple," said James E. Burke, chairman of Johnson & Johnson, which manufactures Tylenol.

Burke said he opposed a nationwide recall such as the one Johnson & Johnson instituted in 1982, but the company urged stores to remove Tylenol capsules from their shelves.

Authorities moved quickly to do just that. By late today, 17 states and the District of Columbia had banned the sale of the capsules, and at least 15 states asked stores to take them off the shelves.

Johnson & Johnson said it would give a refund, or Tylenol tablets or "caplets" (coated pills), to consumers wishing to exchange their Tylenol capsules. The company also suspended production of the medication in capsule form.

In another echo of the 1982 Chicago case of poisoned Tylenol, a New Rochelle man admitted writing a letter to local police claiming to be the "Number Two Tylenol Killer" and demanding $2 million, law enforcement authorities said today. But they said they do not consider him a suspect in the poisoning of Diane Elsroth, 23, of Peekskill, who died Saturday after taking Extra-Strength Tylenol capsules laced with cyanide.

Secret Service agents found a copy of the letter in the home of Dewitt T. Gilmore Jr., 21, when he was arrested Thursday on an unrelated credit-card fraud charge, along with his father, Dewitt T. Gilmore, 55, and a friend, Daffodil Graham, 18.

The original letter to the Bronxville police arrived Thursday, Chief Carl H. Steinmuller said. The handwritten letter stated that more capsules, as well as a case of orange juice, would be poisoned if the sender did not receive $2 million within 72 hours.

The younger Gilmore confessed at his arraignment on the credit-card fraud charge to having written the letter, Steinmuller said.

Secret Service agents had been tailing the Gilmores since last month and were familiar with their movements, Steinmuller said. No charges had been filed against Gilmore as of tonight in connection with the letter.

The tainted capsules that killed Elsroth were traced to a bottle sold at Bronxville's A&P. A second bottle, with five cyanide-laced capsules, was found Thursday in a batch taken off the shelves at a Woolworth store around the corner on this small village's main shopping street.

The tainted medication was the first such instance since the 1982 deaths of seven persons in the Chicago area who were poisoned by tainted Tylenol capsules. The Chicago killer was never found, but James Lewis, one of dozens of suspects questioned by Illinois authorities, was convicted of attempted extortion for sending a letter to Tylenol manufacturers demanding $1 million "to stop the killings."

In 1982, Johnson & Johnson offered a $100,000 reward identical to the one announced today. It has gone unclaimed in Chicago.

The company's stock fell four points on the New York Stock Exchange in today's trading.

The fact that the cyanide from the capsules sold in the A&P came from the same source as that from the Woolworth capsules has led investigators to discount the existence of a copycat killer. The A&P capsules were manufactured in Fort Washington, Pa. The Woolworth capsules were made in Dorado, Puerto Rico, making it improbable that the capsules were altered before distribution.

U.S. Food and Drug Commissioner Frank E. Young said his agency still considered the poisoning to be a "localized matter." In the FDA laboratory in Brooklyn tonight, technicians were working around the clock, testing more capsules from southern Westchester County. Other bottles ordered off the shelves in New York state are being held.

"We're starting at the original location and surrounding areas and then we'll go broader from that," Young said.

In the square-mile village of Bronxville, a wealthy, wooded enclave a few miles north of the Bronx, citizens recoiled from the notoriety. Television cameras were stationed outside the Woolworth store, and reporters roamed Pondfield Road where boutiques, a French restaurant and several antique stores stand near the site of the mansion where John F. Kennedy lived as a boy.

"It's shocking," said Helen Boyle as she rang up sales at Woolworth's. "Nothing like this has ever happened in Bronxville."

At Studio One, a beauty salon owned by John Notarnicola, the mood was somber. "We're devastated," said the receptionist. Notarnicola's wife, Harriet, bought the Tylenol that poisoned Elsroth, the girlfriend of the Notarnicolas' son Michael.

"The question is, do we have another killer loose?" asked police Chief Steinmuller.

"This is a low-key community," he said. " . . . The only killing we ever had was a bank robber shot in 1968."

"I wouldn't be unhappy if this had happened somewhere else," Mayor William J. Murphy said today.