“That’s when we decided to start our plans to make entry and try to rescue the rest of the children,” Chief Mina said, adding that the episode had “a very tragic and sad ending.”

Image Gary Lindsey Jr., who the police say shot an officer early Monday, is believed to have killed four children during a hostage standoff, then himself. Credit... Volusia County Corrections

“Our hearts go out to the families of those four children,” Chief Mina continued.

The episode began about 11:45 p.m. Sunday when Orlando police officers responded to a call at an apartment complex just two miles north of Universal Studios Florida. A woman had contacted officers from elsewhere, telling them that her boyfriend, Mr. Lindsey, had battered her after an argument, the authorities said.

When officers confronted Mr. Lindsey, he opened fire, striking Officer Kevin Valencia, who suffered what Chief Mina called a “very serious and significant injury.” At least one officer fired back.

As of early Tuesday, Officer Valencia, who has been with the department since 2016 and is in his late 20s, remained in critical condition, Chief Mina said.

Mr. Lindsey then barricaded himself in his apartment along with the four children.

Over the next 21 hours, the authorities urged Mr. Lindsey to release the children. Negotiators spoke with him several times throughout the standoff, and had made contact with him as late as about 8:30 p.m. on Monday, just before SWAT officers entered the apartment.