A batch of heroin believed to have killed eight people in Western Massachusetts over the past week might have been exceptionally deadly because of its purity, according to Springfield police.

Investigators in that city, where 9,000 bags of the heroin were seized and one of the deaths occurred, do not believe the drugs had been laced with fentanyl, a powerful painkiller linked to hundreds of opioid deaths in the state, said police Sergeant John Delaney.

Instead, Springfield police believe that the purity of the product, which its distributors stamped with a “Hollywood’’ logo, might have killed its victims, Delaney said. Four fatalities in Chicopee and three in Holyoke also have been linked to the batch.

“When people buy it and use it, they don’t take into account it could be higher purity,’’ said Patrolman Michael Wilk, the Chicopee police spokesman.

A possible link between the deaths and “Hollywood’’-stamped heroin was underscored Saturday night when Springfield police seized the 9,000 bags and confiscated $20,000 in cash.

Four people from the region — one each from Springfield, Holyoke, Chicopee, and North Adams — were arrested in that case on heroin trafficking, possession, and distribution charges. Most of the heroin was discovered stashed in a fake automobile bumper.

State Police are analyzing the seized heroin, and state investigators have not ruled out that the opioid is tainted.

“We are still awaiting tests from our lab to determine the composition of the heroin. It is possible it could be cut with a high level of fentanyl, or that the apparent increased toxicity is caused by some other factor. We will have a better idea when testing is done,’’ said David Procopio, spokesman for the State Police.

The hardest-hit city was Chicopee, where four fatal overdoses occurred between last Wednesday and Monday. Wilk, the police spokesman, said authorities could not recall a similar large cluster of opioid deaths in Chicopee, a city of 56,000 people about 5 miles north of Springfield.

“It could be that where this was a brand-new batch . . . it could have been too fresh and too new for word to have gone around’’ about its lethal potency, Wilk said. “It’s horrible.’’

The Hollywood logo was discovered on packaging discovered in two of the Chicopee deaths, Wilk said. “As soon as we knew this was happening, we put it on our Facebook account,’’ he added.

Delaney, the Springfield sergeant, said the Hollywood heroin most likely made its way to the region from New York City, where a distributor is stamping it with the logo as a marketing tool to separate the drug from competitors.

Its potency could be one way to build a base of new customers in the area who are attracted by its strength, Delaney said. But with enhanced potency — particularly in the recent batch — comes increased danger.

That danger can be compounded during quick, imprecise preparation that often marks the retail heroin trade.

“This is done in somebody’s kitchen or back room, and they might not be knowledgeable and oftentimes make mistakes because they don’t cut it enough,’’ Delaney said. “Anything over 98 percent pure will stop the heart of a veteran drug addict.’’

No overdose deaths have occurred in Springfield since the weekend drug bust, Delaney said, and Chicopee police said their recent warnings on social media appear to have made a difference.

“As far as we know, we did not have any more ODs or deaths, which is a good sign, after the public became more aware of how dangerous this was,’’ Wilk said.

The Springfield arrests will not eliminate heroin from the area, Delaney said, because “other suppliers are picking up the slack.’’

The four suspects charged in the Saturday drug bust are not cooperating with authorities, the sergeant said. But as word of the danger circulates, he said, more addicts probably will dilute their heroin so that one potentially deadly dose becomes two or three nonfatal hits.

“They won’t run away from it. They’ll want to cut it themselves,’’ Delaney said. “Heroin is not going to go away because we got those bags off the street.’’

Brian MacQuarrie can be reached at brian.macquarrie@globe.com.