VERNON, Conn. -- Investigators are trying to determine what caused a house to explode in Connecticut, injuring seven people, including four children, and sending debris flying.

Police, fire and rescue crews responded to the home in Vernon Thursday afternoon. Three of the victims were outside when they arrived, while four more remained trapped in the rubble. Crews dug through what was left of the house to pull them to safety.

"Those four people have been rescued with varying degrees of injury, said Vernon police Lt. William Meier.

Four children, all under the age of 16, were among the victims. They were taken to Connecticut Children's Medical Center, where one was treated and released. The other three remained hospitalized early Friday morning.

"Each of these patients sustained significant, but no immediately life-threatening injuries," Dr. Brendan Campbell told reporters gathered outside the hospital.

A 7-year-old boy appeared to suffer the most serious injuries, according to police.

Two adults were taken to Hartford Hospital. Another adult at the scene refused medical treatment.

The force of the blast blew the front door of the home across the street into a neighbor's yard. Glass and other debris littered the roadway.

"I never heard anything like it. It wasn't just a vibration. The house shook," neighbor Alan Fisher told the Hartford Courant.

Connecticut State Police troopers and K-9 teams assisted Vernon police and fire.

Gov. Dannel P. Malloy visited the scene late Thursday to meet with emergency management officials. He said he would continue to monitor the situation closely.