The bodies of Sarah and Tyler Schmidt, both 42, and their daughter Lula were found in their tent in the Maquoketa Caves State Park.
The couple’s nine-year-old son survived the attack, a neighbour said.
Police believe the suspected shooter, a 23-year-old man, then turned the gun on himself. His body was also found in the park.
An investigation into the incident is ongoing, and the Maquoketa Caves State Park is an active crime scene, a statement from Mitch Mortvedt, Assistant Director of the Iowa Department of Criminal Investigation, said.
Mr Mortvedt told local newspaper, The Des Moines Register, that there had been no reports of an earlier altercation between the suspect and victims.
The suspected shooter has been named as Anthony Orlando Sherwin, who was also staying at the campsite.
The victims’ bodies were found on Friday morning, and emergency services were called at 06:23 local time (12:23 BST).
The park was evacuated, and Sherwin, known to be armed, was found to be missing. Following an air search, his body was found in the park, some way from the campsite.
The Schmidts were from Cedar Falls, also in Iowa, which has been rocked by the news.
Cedar Falls mayor Rob Green wrote in a Facebook post that just this week he had been working with Sarah Schmidt, who worked at the local library, on a project.
“Like many of you just hearing the news, I’m devastated,” he said.
“I knew Sarah well, and she and her family were regular walkers here in the Sartori Park neighbourhood.”
Mr Green’s post received many comments expressing shock and grief, as well as sympathy for the surviving child.
A GoFundMe page set up to help the nine-year-old had raised almost $150,000 (£125,000) by Sunday evening.
The page organiser said she was Ms Schmidt’s cousin, and that the boy was “surrounded by family and friends who are supporting him as best we can”.
Local officials said autopsies would be carried out on all four of the bodies over the weekend.
The park is expected to be closed until at least Thursday.
Kayla Lyon, Director of Iowa Department of Natural Resources, said the “long-standing tradition of enjoying Iowa’s natural wonders” had been shaken, but would continue.
