A “lava bomb” explosion from the Kilauea eruption’s ocean entry point punctured the roof of a tour boat, injuring 23 passengers early this morning, according to the Hawaii County Fire Department.

The Lava Ocean Tours Inc. vessel was within the Coast Guard’s 300-meter (984-foot) safety zone at the time of the lava explosion shortly after 6 a.m., state and county officials said.

Fire officials said the tour boat “Hot Spot” arrived about 500 yards offshore of the lava entry point and proceeded to get closer until it was about 200 yards, or 600 feet from the shoreline.

>> See more photos of the boat damage from the ‘lava bomb’

The boat then turned out to sea when “an explosion occurred near the shoreline hurling hot lava rocks toward the boat and injuring several passengers,” fire department officials said.

The boat returned to Wailoa Harbor in Hilo at about 7 a.m. today where medics and firefighters responded.

Three people were taken by ambulance to Hilo Medical Center, while a fourth went to the hospital in a personal vehicle, fire officials said. Two of the three passengers were in stable condition. The third passenger described to be a woman in her 20s is in serious condition with a fractured femur.

The fire department said nine passengers drove themselves to the hospital and medics treated 10 people at the scene for minor burns and abrasions.

Shane Turpin, the owner and captain of the “Hot Spot”, told the Associated Press that he never saw the explosion that rained molten rocks down on top of his boat.

He and his tour group had been in the area for about 20 minutes making passes of the ocean entry about 500 yards offshore, Turpin said. He didn’t observe “any major explosions,” so he navigated his vessel closer, to about 250 yards away from the lava.

“As we were exiting the zone, all of a sudden everything around us exploded,” he said. “It was everywhere.”

Turpin said he had no idea just how big the blast was until he saw video of the event later on shore. “It was immense,” he said. “I had no idea. We didn’t see it.”

He said most of the injuries were minor, but that he had visited one woman who sustained serious injuries in the hospital.

In addition to the punctured roof, the boat sustained damage to a railing.

The Coast Guard and the state Department of Land and Natural Resources’ Division of Conservation and Resources Enforcement are investigating to determine the exact location of the vessel at the time of the explosion. “We’re working to ascertain facts,” said Coast Guard Petty Officer 3rd Class Amanda Levasseur.

A large volume of lava from fissure 8 in the lower East Rift Zone continues to enter the ocean at Kapoho since the May 3 eruption.

The Coast Guard had implemented the safety zone surrounding the ocean entry of lava on the island’s southeast side to protect people and vessels from potential hazards of the active flow.

Entry into the zone which encompasses all waters extending 300 meters or 984 feet in every direction around the ocean-entry point of the lava flow is prohibited.

According to Coast Guard Spokesman Petty Officer 3rd Class Matthew West, the Hot Spot vessel operated by Lava Ocean Tours, Inc., however, was one of the vessels granted permission last week by the Coast Guard to gain entry into the safety zone, as close as 50 meters from shore.

As of right now, West said entry into the 300-meter safety zone by all vessels is prohibited.

Visitor Vim Mahadevan, 48, of Sherman Oaks, Calif., who is on vacation on Hawaii island with his family, was on a separate lava tour boat when he saw the affected vessel. Mahadevan said they left the Wailoa boat ramp at about 4 a.m. today, the same time as the Lava Ocean Tours vessel.

Mahadevan was seated in the front row of the boat when he saw the other vessel in the water at the time of the explosion. There were rocks and ash that landed on that boat, he said.

He estimated the other boat was less than 500 feet offshore. “They seemed really really close,” Mahadevan said. “We were 1,000 feet away (offshore). Those guys were much closer.”

He described the lava bomb that struck the tour boat as a fireball that went up about 20 to 30 feet into the air. A large plume of billowing white smoke soon followed.

When Mahadevan saw the damaged lava boat, he said, “We were hoping that no one was injured.” He estimated about 48 to 50 were aboard the vessel.

Mahadevan then observed a second explosion in the area. “The second one was even larger,” he said.

No vessels were impacted by the second explosion.

The Associated Press contributed to this report.