Several of the monkeys had escaped when a lorry carrying a trailer of 100 of them crashed on Friday.
By Saturday, one of the cynomolgus macaque monkeys was still at large - with authorities in the state warning the public not to approach them.
However, on Sunday, a spokesperson for the federal Centers for Disease Control and Prevention said they had all been accounted for.
The spokeswoman, Kristen Nordlund, said three of the monkeys were dead after being “humanely” euthanised, but did not elaborate on why they were euthanised or how the other monkeys came to be found.
The shipment of monkeys had been en-route to a quarantine facility after arriving Friday morning at New York’s Kennedy Airport from Mauritius, police said.
The collision occurred Friday on a state highway near an Interstate 80 exit in Pennsylvania’s Montour County, Trooper Andrea Pelachick told the local Daily Item newspaper.
It remains unclear where the monkeys were headed or the type of research they were apparently destined for.
However, the species are often used in preclinical toxicology studies.
Police had earlier previously urged people not to look for or capture any monkey, with troopers tweeting: "Anyone who sees or locates the monkey is asked not to approach, attempt to catch, or come in contact with the monkey. Please call 911 immediately."
Trooper Lauren Lesher had said the concern was "due to it not being a domesticated animal and them being in an unknown territory. It is hard to say how they would react to a human approaching them."
Officers used thermal imagery and helicopters in a bid to locate the missing animals.
A witness to the collision, Michelle Fallon, told local media of her shock at seeing the monkeys after she went to help the pickup driver and a passenger.
The drivers of both trucks weren’t harmed, and a passenger was taken for treatment for suspected minor injuries, said a state police crash report.