OSHKOSH, Wis. (WFRV) — Four people are dead and two are hospitalized after two separate crashes on Saturday. Both incidents are connected to an annual air show in Wisconsin, according to officials.

The first crash happened shortly after 9 a.m. A T-6 Texan aircraft crashed into Lake Winnebago near Oshkosh, where the Experimental Aircraft Association is holding its annual fly-in convention, EAA AirVenture.

Both people aboard the aircraft died, according to EAA officials.

A few hours later, a Rotorway 162F helicopter and ELA 10 Eclipse gyrocopter collided in mid-air at the south end of the EAA AirVenture flight line at Wittman Regional Airport, killing two and injuring two others. Both of the injured are in stable condition, according to EAA.

The Oshkosh Fire Department said one of the aircraft landed on top of a parked plane, but did not clarify which.

The National Transportation Safety Board is investigating both accidents and has not provided a cause for either.

“Naturally, with the mid-air collision, somebody was in the wrong place at the wrong time,” EAA director of communications Dick Knapinski said. “How that occurred and why that occurred is what the NTSB will investigate and, from then, lessons will be learned as to what aviators can do to keep themselves safer.”

The investigation will likely take months to reach its conclusion.

“NTSB investigations typically take between a full year to 18 months to complete,” Knapinski explained. “They will have a preliminary investigation within the next week or so and then put out those findings. But the probable cause will probably take a year or more to determine by the board.”

The factors the NTSB will examine in its investigations include weather conditions, pilot experience, aircraft condition, flight pattern, and communication between ground and flight operations to determine the cause, which could be pilot error, a mechanical issue, or another reason, according to Knapinski. He described the weather as “mild” and likely not a contributing factor.

“Having accidents like this in the singular is very rare,” he said. “Having two fatal accidents take place on the same day is exceedingly rare.”

These are not the first incidents regarding aircraft at or en route to EAA this week. On Thursday, a single-engine plane bound for EAA AirVenture crash-landed in a cornfield in Green Lake County, hospitalizing two people.

On Tuesday, Knapinski said there was a “forced landing” east of the north/south runway at EAA AirVenture. The pilot was the only person onboard and was not injured.

Last Saturday, a single-engine ERCO Ercoupe headed to EAA AirVenture crashed in Fond du Lac County, injuring the pilot.

The Associated Press contributed to this report.