The overdoses were initially feared to stem from a batch of synthetic marijuana, also known as K2, or Spice, that had possibly been laced with an opioid.
“Whether it be a synthetic opioid or fentanyl, we are not sure at this time until we have confirmation on the analysis from the D.E.A.,” Rick Fontana, the New Haven director of emergency operations, said.
However initial tests found the substance to be pure K2 and did not contain any additives, NBC quoted officials as saying.
The United States is grappling with a surging epidemic of opioid abuse - including painkillers like OxyContin and street drugs like heroin.
A new report estimated a record 72,000 Americans died last year from drug overdoses, a rise of around 10 percent, the Centres for Disease Control said on Wednesday.
Drugs, particularly fentanyl, are now deadlier in the US than either HIV, car crashes or guns, the report said.
Overdose deaths involving synthetic opioids rose sharply, while fatalities from heroin, prescription opioid pills and methadone fell, according to the C.D.C. estimates.
In October, Donald Trump labelled America’s opioid epidemic a “public health emergency”, giving officials extra powers to tackle the crisis.
However Mr Trump fell short of announcing a full “national emergency” – as he had previously suggested - which would have meant new central funding.