Three homes in the New Hampshire towns of Enfield and Orange were searched late Sunday and early Monday where police seized evidence related to the cases, police said.
Three juveniles and one adult, who have not been publicly identified, will be charged later this week in both states, police said.
Police have not said publicly what they felt motivated the bombings. No injuries have been reported.
"During the course of this investigation suspects in the Upper Valley pipe bombings have been identified," said Enfield Police Chief Richard Crate. "The investigators are confident they have brought to a conclusion at least eight incidents between the state of New Hampshire and Vermont."
Crate said the break in the case came Saturday night when police stopped a car for a minor traffic violation and spotted a canister of gunpowder in the back. He did not identify the two youths — ages 17 and 18 — involved because they have not been charged, but said computer searches using their names turned up online discussions of their alleged activities.
He said that led to three searches, two in Enfield and one in Orange. The searches turned up steel pipes, screw-on caps to seal them, gunpowder and fuses.
Crate said the police departments involved — including state police in both states — now estimate there have been 10 to 12 explosions in the past week against targets including mailboxes and portable toilets.
Investigators are continuing to collect information about the case. Even though the suspects are not in custody, Vermont State Police Sgt. John Flannigan said police did not believe there was an ongoing threat to the public.
Flannigan wouldn't talk about the suspects' possible motive.
"We're not going to release it, even if we do know the motivation, until there are charges filed," he said.
Police have not offered an estimate of the damage from the bombings.
On Friday, Vermont State Police discovered that a bomb had damaged a pay phone near the Vermont Law School in South Royalton. That bombing was followed by arson fires on Vermont Route 14 in Sharon, on town roads in Royalton and Barnard, police said.
On Saturday night, Vermont State Police received a report of an explosion and found a portable toilet behind the Royalton school, which was destroyed in that blast.
Flannigan said police don't know if the bombings and the arson fires were related.