Twenty-six people, most from the Merrimack Valley, have been indicted as part of a heroin trafficking ring that officials described as an “extremely violent and very organized’’ cartel, federal authorities said Thursday.

Sixteen face charges of conspiring to distribute more than a kilogram of heroin, while others face lesser charges of conspiring to distribute heroin, according to the New Hampshire US attorney’s office and the Drug Enforcement Administration.

One person, Eric Guerrero, 30, of Lawrence, was charged with two counts of fentanyl distribution and conspiracy to distribute fentanyl. Guerrero is ­also known by the names “Tony’’ and “Edward Garcia.’’

Fentanyl is a synthetic opioid with the same biological effects as heroin but potentially hundreds of times more potent, according to the DEA.

Twenty-one of those indicted were from Massachusetts — mostly from Lawrence and Haverhill, but also Lowell, Methuen, and Salem.

The investigation “was about 18 months and included approximately six months of wire taps on five phones,’’ New Hampshire US Attorney Emily Gray Rice said. “These incidents are going to have a significant impact; however, the problem is extremely pervasive.’’

The defendants, who are accused of distributing the drugs mainly in northern Massachusetts and southern New Hampshire, were arrested between Oct. 12 and Tuesday, or were already in custody on other charges, Rice’s office said. Two people remain at large.

Michael J. Ferguson, special agent in charge of the DEA’s New England Field Division, said the agency “is addressing the threat of heroin and fentanyl, both internationally and domestically.’’

“We prioritize our resources by identifying, targeting, and destroying these extremely violent and very organized cartels,’’ he said.

Ferguson said the DEA has “effectively dismantled the distribution and facilitation elements’’ of this organization, which spread heroin and fentanyl in New Hampshire and throughout New England.

The DEA worked with Homeland Security, Massachusetts and New Hampshire State Police, and local police departments on the investigation.

McGuinness can be reached at dylan.mcguinness@globe.com.