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This story includes graphic descriptions of crimes committed against an infant.

Texas executed Travis James Mullis on Tuesday for killing his infant son in 2008, making him the fourth inmate executed in the nation's busiest death penalty state this year and the 16th in the nation.

Mullis was executed by lethal injection and pronounced dead at 7:01 p.m., according to Amanda Hernandez, a spokesperson for the Texas Department of Criminal Justice.

Mullis was convicted of murdering his 3-month-old son Alijah, who was molested, stomped to death and abandoned at the Galveston’s Seawall, a popular tourist destination just south of Houston. Mullis, who was 21 at the time, said he had reached a "breaking point" the day he killed Alijah after the baby wouldn't stop crying.

Mullis was executed less than an hour after Missouri executed Marcellus Williams despite strong questions surrounding his guilt. Mullis and Williams are among five inmates scheduled for execution in the U.S. in a six-day period between Sept. 20 and 26. Freddie Owens was the first to be killed on Friday in South Carolina.

Here's what to know about the Mullis' execution − including his last words − the case and the victim.

'It was my decision that put me here,' Travis Mullis says before execution

Travis Mullis used his last words to thank and apologize to loved ones, including the mother of his child and the victim's family, according to a transcript obtained by USA TODAY.

He thanked everyone, but specifically shouted out all of his friends, pen pals and all the people in his corner inside and outside, even on Death Row, who "accepted me for the man I became during my best and worst moments."

"I want to thank the field ministers, the Warden, and the correctional staff for all the changes being made across the system," Mullis said. "Even the men on Death Row, to show it is possible to be rehabilitated, and not deemed a threat and not the men we were when we came into the system. We have changed. We are not the same."

Mullis said that he took the "legal steps" to expedite his death sentence, which he referred to as "assisted suicide" and that he doesn't regret it. But that he did regret killing his son, Alijah.

"I do regret the decision to take the life of my son. I apologize to the mother of my son, the victim's family," he said. "I have no ill will towards the court, the judicial system, the prosecution or the execution protocol. The morality of execution is between you and God ... It was my decision that put me here. I'm ready Warden.”

'Rest in Peace,' lawyer says

Shawn Nolan, Mullis' defense attorney, said in a statement that his client has "always accepted responsibility" for the awful crime he committed.

"Texas will kill a redeemed man tonight. He never had a chance at life being abandoned by his parents and then severely abused by his adoptive father starting at age 3, "Nolan wrote. "During his decade and a half on death row he spent countless hours working on his redemption. And he achieved it."

The Travis that Texas wanted to kill is "long gone," he said.

Travis Mullis' judgement has 'finally been carried out'

Jack Roady, Galveston County criminal district attorney, said in a statement that Tuesday marked the "long-awaited fulfillment of a verdict rendered by a jury who heard all of the evidence."

"We are also grateful for the diligent investigation of the Galveston police officers who confronted the most disturbing facts and images possible in solving this case," Roady said. "My deepest thanks go to Kayla Allen, Donna Cameron, and the late Lyn McClellan – the prosecutors who faithfully carried this tragic case to trial and ensured that justice was done for Alijah Mullis."

Kayla Allen, first assistant district attorney, noted that the jury's verdict was "affirmed by 13 years of post-conviction review by higher courts."

"Some acts are such egregious violations of not only the law, but of civilized standards, that society’s only appropriate response is to impose the ultimate penalty on the wrongdoer," she said, noting that Alijah would have "celebrated his 17^th birthday next month."

'Extraordinarily beautiful' baby gone too soon

Carolyn Entriken, Alijah's grandmother, died two years before Mullis was executed and 14 years after her grandson.

To her, Alijah, was the most "extraordinarily beautiful" baby, Entriken told a court, according to a March 2011 transcript obtained by USA TODAY.

“He had steel blue eyes, cute little reddish hair," Entriken said. "I know all babies are beautiful ... He just was very precious."

Entriken got the chance to visit her grandson a couple months after he was born and one month before he was killed, making her way to Texas from northern New Jersey. Mullis seemed "very loving and caring" during the first visit, telling the court that they "looked like a young family out on an outing.” 

Entriken planned to visit again soon because she "wanted to come back and see Alijah."

“I didn't want too much time to go by where he was growing up without my seeing him," she said.

What was Travis James Mullis convicted of?

Mullis had hit his "breaking point" when he killed his son, telling Philadelphia police four days after the murder that he couldn't get him to stop crying and thought that the only way to get him to quiet down was to kill him, according to court records.

He had spent the night before the murder bickering with then-girlfriend and mother of his child, Caren Kohberger, after he had tried to get his roommate's 8-year-old daughter to pull her pants down, court records say.

The couple was afraid that they would be thrown out of the house and that Mullis would mentally relapse and act out on his impulses again. The young family had also run out of money at that point, hoping that Mullis could get a job before they had to start paying rent, court records say.

That morning, Mullis said he had to get away to clear his head. He took Alijah with him and drove south toward Galveston, where he pulled over in a secluded area of the seawall. But then, his son started to cry, and he couldn't get him to stop, he told police.

Mullis molested and choked the infant before he pulled Alijah out of the car, put him on the ground and stomped on his head, court records say. He flung the car seat and his son's body toward the other side of the seawall then fled the state, USA TODAY reported.

He turned himself in a couple days later, offering a detailed confession to authorities in Philadelphia. Mullis was extradited back to Texas, convicted of murder and sentenced to death in March 2011.

This story has been updated to add new information.