A seven-month investigation into heroin trafficking has resulted in the arrests of five reputed dealers as authorities attack a recent spike in heroin use and overdoses in the northern part of Westchester County, District Attorney Janet DiFiore said Wednesday.

The arrests of Nathaniel "Happy" Graham, 35, Vincent Altieri, 21, and Michael Robles, 31, all of Cortlandt, together with Alonzo Thompson, 51, and Stephen Agosto, 30, both of Croton-on-Hudson, came about as a result of an investigation conducted by the Westchester District Attorney's Narcotics Initiative along with the State Police, Croton-on-Hudson police and Harrison police, DiFiore said.

While the rash of overdose deaths in the last year has spurred increased efforts by law enforcement agencies in the leafy hamlets of Westchester's more rural northern tier and in largely rural Putnam County, connecting dealers to specific deaths has been a difficult task, law enforcement authorities say.

"It's awfully hard to tell where a habitual user who dies from an overdose got his last bag when there are so many options for them," one law enforcement source told Newsday.

On March 20, two Cortlandt brothers overdosed on heroin in their Wren Drive home. George Landers, 25, died from the overdose; his 27-year-old brother Michael survived after being rushed to Phelps Memorial Hospital.

That same day, police arrested Eric Rabe, 23, of Cortlandt Manor, who allegedly was caught making a heroin sale on Westbrook Drive.

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On March 21, FBI agents and Peekskill police conducted a major roundup of reputed heroin dealing gangs, charging 17 people in Peekskill with involvement in the drug peddling rings.

Federal, state, county and local law enforcement agencies in northern Westchester County and Putnam County have been coordinating investigations into heroin sales since four men died of heroin overdoses within the same week in early December 2012. Police began working with the Putnam County Health Department after those deaths were reported.

In early March, Putnam County sheriff's deputies seized heroin with a street value of $6,000 from Grigory Yarmarkov, a Southeast man described as a "midlevel supplier" to local drug dealers. Deputies found Yarmarkov with 450 heroin packets, authorities said.

In January, sheriff's deputies arrested 23-year-old Franyelis Ramos, a Southeast man who police said moved 1,000 packets of the drug per week.

And on Dec. 19, State Police and local cops nabbed four people from Somers and Yorktown -- including Peter Muir, 25, and his sister Alora Muir, 19 -- and charged them with criminal possession of a controlled substance with intent to sell.

In the case discussed Wednesday, the defendants were spotted making multiple heroin sales to an undercover officer in several municipalities, investigators said.

Authorities said the accused dealers are not connected to one another.

Altieri was arrested on March 21, Nathaniel Graham on March 27, Robles on May 2 and Thompson and Agosto both on May 3, authorities said.

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Police executed search warrants both at Agosto's 168 Grand St. home in Croton -- seizing oxycodone -- and at Altieri's 42 Locust Ave. home in Cortlandt, where they seized nearly an ounce of cocaine and 500 "$10" bags of heroin packaged for sale on the street, as well as $3,000 in cash.

The investigation is continuing.

Graham, Altieri, Thompson and Agosto have been indicted by a Westchester County grand jury on charges of felony third-degree criminal sale of a controlled substance, among other charges. They face up to 25 years in prison if convicted.

Robles was charged in a felony criminal complaint with third-degree criminal sale of a controlled substance.

Lawyers for the five defendants could not be located.

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The investigation is continuing, DiFiore's office said.

All the defendants are being held without bail at Westchester County Jail in Valhalla.