Fifteen Bloods gang members and associates were charged Thursday with operating a "sophisticated" Long Island heroin ring that laced some of the drug with the deadly additive fentanyl, authorities said.

The ring, based in Nassau County, is also accused of running a lucrative dogfighting racket, in which pit bulls were bred and trained to attack one another in high-stakes matches.

Gang members would stitch the mouths of "bait dogs" shut so other pit bulls could maul them during "training" sessions, officials said.

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"Those arrested in this operation represent some of the worst of society," said Nassau Police Commissioner Thomas Krumpter, who announced the arrests with County Executive Edward Mangano and acting District Attorney Madeline Singas. "They deal in death and the destruction of lives."

More than 600 bags of ultra-potent heroin and 11 dogs, including two newborn puppies, were seized in the bust dubbed "Operation Blood Sport" by prosecutors and police.

Police said the crew also peddled opioid pain pills and other prescription drugs.

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Authorities could not immediately provide a specific street value or weight of the seized heroin, but said the trafficking operation ranked among the largest in the county.

After a five-month investigation, authorities executed search warrants at five locations in Roosevelt and Freeport on Wednesday.

Officials said the "widespread conspiracy" was overseen by Bloods crews based in Freeport and Roosevelt. The ring also operated in Lynbrook, Baldwin and other areas, selling to hundreds of customers, some of whom were lower-level dealers.

Detectives are investigating whether the heroin came from Mexico-based drug cartels, which export most of the heroin sold on Long Island.

Officials said the ring exemplified three significant threats facing Long Island: drug-dealing street gangs, fentanyl-laced heroin and animal abuse.

The region's myriad street gangs, including Bloods, Crips and MS-13, claim thousands of members and often have a hand in both drug trafficking and dogfighting, officials said.

Singas said the gang members used sleds, chains, poles and other restraint tools to cruelly train fighting dogs.

Some animals were fed muscle-building supplements to improve performance, with thousands of dollars wagered on individual dogfights, officials said.

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Nassau prosecutors, gang and narcotics cops, and investigators with the American Society for the Prevention of Cruelty to Animals' national Blood Sports division were involved in the probe.

The defendants were arraigned Thursday on charges ranging from conspiracy and drug possession to dogfighting and weapons possession.

Authorities identified three Freeport men as the ringleaders: Shaheem Allen, 23; Daquan McLean, 23; and Kwame Reaux, 25.

Others facing charges are: Ava Green, 22; Nehiem West, 17; Arivita Fulton, 45; Anthony Reddick, 54; Darren Devore, 26; and Rahiem West, 17, all of Freeport; Travis Mouzon, 35, of Roosevelt; Sheron Davis, 27, of Uniondale; Christopher Nelson, 24, of North Babylon; Terrance Collins, 23, and Ty Mitchell, 19, both of Amityville; and Kara Jeter, 28, of Queens.

Lawyers for the accused could not immediately be reached for comment.

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Fentanyl, found in some of the heroin allegedly sold by the crew, has been linked to at least 42 overdose deaths on Long Island over the past 15 months.

Local heroin dealers increasingly lace their product with the drug to give it an extra kick, officials said. Some users inject fentanyl-laced heroin unknowingly, while others seek it out for a more powerful high.

Fentanyl is the most potent opioid available for use in medical treatment, 50 to 100 times stronger than morphine and 30 to 50 times more powerful than heroin, authorities said.

Opioid overdoses killed at least 341 people on Long Island in 2014, records show. It's unclear how many of those deaths were caused by a heroin-fentanyl combination, as that statistic is not tracked by local medical examiners. The surge in fentanyl-related overdoses locally reflects a national trend, officials said.

In March, the Drug Enforcement Administration issued a nationwide alert about fentanyl-laced heroin, with agency officials calling the overdose rate "alarming" and saying it represented a significant threat to public health.

During the past two years, officials said they have seen a major increase in fentanyl-related drug seizures across the nation.

Crime labs reported 3,344 fentanyl submissions nationally in 2014, up from 942 in 2013, according to the National Forensic Laboratory Information System, which compiles drug-testing data.