The house on Elbert Street in northeast Houston was pitch-dark last Thursday when an FBI SWAT team assembled outside in the early morning hours.

One group of agents stalked up the gravel driveway toward the front of the one-story residence. Two other agents approached through the backyard.

Bound in gray duct tape inside one of the backrooms was Ulises Valladares, a 47-year-old man who had been kidnapped at gunpoint the previous morning. Authorities said he was being held for ransom by people claiming they belonged to a Mexican drug cartel.

Just after 3:30 a.m., agents in front called out “FBI!” and threw flash-bang grenades to stun the captors, while the pair in the rear tried to smash through a window and rescue Valladares, according to authorities.

But in the chaos that followed, an agent fired his rifle, striking Valladares and killing him in what Houston police Chief Art Acevedo said Tuesday was a tragic mistake.

“A well-intentioned effort to rescue a man that had been kidnapped ended in a tragic outcome,” Acevedo said. “I don’t wish that upon anybody.”

Speaking at a news conference, Acevedo gave the first detailed account of the botched operation since Valladares was snatched from his home at gunpoint last week. The incident made national headlines after police acknowledged that Valladares had been killed by an FBI agent during the raid but did not explain how the shooting unfolded.

Three people have been charged in connection with Valladares’ kidnapping: Nicholas Chase Cunningham, 42; Sophia Perez Heath, 35; and Jimmy Tony Sanchez, 38. All three were charged with aggravated kidnapping. Cunningham and Sanchez were also charged with aggravated robbery, as The Washington Post has reported. It was not clear Tuesday if they had retained attorneys or entered pleas.

The agent who shot Valladares has been placed on leave pending an inquiry. He has not been identified, which is standard FBI policy. Acevedo said an FBI task force is investigating and that the agent gave them a voluntary statement.

Perrye Turner, special agent in charge of the FBI Houston field office, said in a statement Tuesday that the agency took the incident “very seriously” but declined to offer additional details.

Valladares was at home in Conroe, Texas, last Wednesday morning with his 12-year-old son, who was getting ready for school, when they heard a knock at the door. When Valladares answered it, two men tackled him to the ground, according to a criminal complaint released by the Montgomery County District Attorney’s Office.

The boy told police the men bound him and his father in duct tape, then ransacked the residence, grabbing a PlayStation, an Xbox and other valuables.

According to the complaint, the intruders told Valladares they were there because his brother owed them $8,000. After a 20-minute tear through the house, they threw a black Nike sweater over Valladares’ head and carried him outside. They left behind the boy, who eventually freed himself with a pair of scissors and called police.

Soon after, a male speaking in Spanish reached Valladares’ brother by phone and demanded $20,000 in ransom money, according to police. The caller said he was with the Gulf Cartel in Mexico, but authorities said that was likely a ploy to intimidate the brother.

Police quickly traced the call to a motel, where they found Cunningham and Sanchez. The suspects directed authorities to the house on Elbert Street, a tree-lined block several miles from downtown Houston.

Acevedo, the police chief, said Tuesday that Houston police cordoned off the surrounding area while FBI agents descended on the property. He said they had received reliable intelligence that Valladares’ was tied up in the backroom.

The lights were out, but the agents decided not to use flashlights because they were worried about inadvertently blinding their partners, Acevedo said. He called the move a “tactical decision.”

When the time was right, the SWAT team sprung. As one group stormed the front of the house, an agent in the back used a breaching bar to break open the rear window.

At some point, the agent lost his grip on the tool and it fell inside, according to the police chief. When he tried to use his M-4 rifle as a substitute, he felt a tug on the barrel. In a panic, he fired two shots, Acevedo said. One hit the eaves of the rooftop. The other struck Valladares, who had been sitting by the window, his hands tied in front of him, according to the police chief.

Valladares died of a gunshot wound in the hospital later that day. Heath, the female suspect, was arrested inside the house.

Acevedo said the agent, fearing for his life, made a “split-second decision” and pulled the trigger intentionally.

“You can imagine when you’re that agent, not being able to see in, worried to lose your rifle. That’s why he ended up discharging it,” he told reporters Tuesday.

The police chief said it was not clear why Valladares grabbed the agent’s gun.

“He was bound, obviously, with his hands in front of him, taped and not able to see in the dark,” he said. “If I had to speculate it would be he was trying to get out of that room.” Acevedo added that “the people that put this set of circumstances into play are people that committed a kidnapping.”

The trio charged in Valladares’ kidnapping could face felony murder charges, depending on the results of the investigation, which Acevedo said is expected to conclude in the coming weeks.

An attorney for the Valladares family attorney, Doug York, hinted that the family is mulling a lawsuit. The FBI either failed to train the agent or the agent himself did something wrong, he told local media.

“I have no earthly clue what went through their mind and why they decided to pull their trigger not once, but twice into a dark room into something where you have no line of sight,” York said. “These kidnappers started the process and the FBI unfortunately finished it the wrong way.”

Follow the Bangor Daily News on Facebook for the latest Maine news.