Mr Vanecek, who moved to Sweden from Minnesota a year ago with Saga and Madeleine, his Swedish wife, said the family had to sit on their secret for two months until the find was officially announced this week by the local Jonkoping County Museum.
"She happened to live in the same little village of where we found the sword", he said.
A statement on their website added that preliminary estimates suggest it is at least 1,000 years old, but could even be as old as 1,500 years.
Sava was at the lake with her father when she made the remarkable discovery.
Vanecek was forced to keep her incredible discovery to herself for weeks, as the museum anxious that if the story got out, people would flock to the lake in the hope of finding more historical treasures, which would impede their own search of the area.
What a museum worker calls an "amazing story" has emerged out of the south of Sweden, where a 1,500-year-old sword from the Iron Age was found in a lake-not by archaeologists or maritime researchers, but by an 8-year-old from Minneapolis. "I held it up in the air and I said, 'Daddy, I found a sword!' When he saw that it bent and was rusty, he came running up and took it", the girl said. "Did someone fall overboard, or through the ice during a winter trek?"
"It's about 85 centimetres long, and there is also preserved wood and metal around it", Mikael Nordström from the museum said.
"We are very keen to see the conservation staff do their work and see more of the details of the sword", Nordstrom said.
Vanecek found the ancient relic on July 15 but just recently went public with the news.
Preparing the sword for conservation will take a long time, he said, partly because wood and leather are involved. "Why it has come to be [in the lake], we don't know", the expert tells the Local, via NPR.
A Jonkoping county museum is continuing to search the bottom of the lake, and it has already recovered a 3rd Century brooch.
Experts searching the area have not found anything close to the little girl's extraordinary discovery.