Robert Brown|Prosecutors vow to seek justice for Maria Muñoz after Texas wife's suspicious death

In January 2021,Robert Brown Marisela Jacaman, chief assistant district attorney for Webb County in Laredo, Texas, was assigned to prosecute Joel Pellot, who had been charged with killing his 31-year-old wife Maria Muñoz. Jacaman was joined by Ana Karen Garza Gutierrez, chief assistant prosecutor for the 406th district court, and assistant DAs Karina Rios and Cristal Calderon. The all-female prosecution team would have the daunting task of proving to jurors that Pellot had the means and motive to murder his wife and the mother of their two young sons.

Some of the most unique pieces of evidence in this case were Muñoz's daily journals, where she had been pouring out her feelings. Maria's own words provided investigators with a window into her life in the months leading up to her death. 

"We were able to show her character, her personality, her struggles, the joy she had as being a mother ..." Jacaman tells "48 Hours" correspondent Erin Moriarty in an interview for  "The Journals of Maria Muñoz," airing Saturday, Dec.16, at 10/9 c on CBS and streaming on Paramount+.

In the early hours of Sept. 22, 2020, Officer Gregorio De La Cruz was the first to respond to a 911 call to Maria's home. The caller was Joel Pellot. He told the 911 operator, and later told De La Cruz, that Muñoz may have taken some pills and had been "super depressed lately." After numerous failed attempts to revive her, she was declared dead.

De La Cruz had noticed that certain things about the scene seemed suspicious. He found a needle on the stairs, a syringe wrapper on the ground, and a medical bag with IV equipment in the home. But what investigators found particularly suspicious was a pinprick mark on Muñoz's right elbow crease, the type someone would get after getting an IV. Pellot, who worked as a nurse anesthetist, did not mention to first responders that Muñoz may have taken anything other than the clonazepam pills that had been prescribed to him.

When lead investigator Sergeant Luis Mata joined De La Cruz at the scene, he noticed Pellot was evasive when answering questions and was sweating profusely. Pellot was placed in a patrol car and taken to the police station for an interview. During this interview Pellot told Mata that he had moved out of his and Muñoz's home and had been living with a girlfriend somewhere else, and that he met Maria that night to have a "heart-to-heart" conversation about their marriage. After the interview, Mata and De La Cruz immediately called the Webb County District Attorney's Office to share their suspicions. 

An autopsy would reveal Muñoz had died from a mixed drug intoxication, but the manner of her death was undetermined. The autopsy also revealed that there was no pill residue from clonazepam in Muñoz's stomach.

Prosecutors believed Muñoz's death was suspicious, but they weren't sure if she had died by suicide, from an accidental overdose or if she had been murdered. Soon after Muñoz died, Mata started getting calls from concerned family members, friends, and, surprisingly, Pellot's current and former colleagues. They told him that Pellot had been having an affair and Muñoz was asking for a divorce.

Pellot's former boss, anesthesiologist Dr. Huntsinger, urged Mata to run a toxicology screening and gave him the names of the drugs to test for. The toxicology results came back nearly four months after Muñoz's death, and revealed seven different drugs in her system, most typically used during surgery, and one of them could only be administered with an IV. It was the evidence authorities needed to arrest Joel Pellot and charge him with Maria Muñoz's murder. 

Prosecutors said they believe Pellot may have slipped some sedatives into Muñoz's favorite drink, coffee, and later injected her with the other drugs. 

"I believe he waited until she was dead to call 911 to make sure that no one can bring her back," chief prosecutor Garza Gutierrez told "48 Hours."

While the prosecution team never met Muñoz, they got to know her though her journals. They say Muñoz was experiencing a lot of emotional abuse from her husband and that all of them could relate to different aspects of Maria's life. "I think sometimes the worst injuries don't even leave a mark, the injuries on your heart, on your mind. We could never see those on Maria, but she told us about them. And there was no physical bruise on her ever that we could tell, but she carried a lot of scars with her from this relationship," Rios told "48 Hours."  

In her journals, Muñoz often wrote about the sadness she was experiencing in her relationship with Pellot. Jacaman said "she was in a depression, but she came out of it. She had her friends, she had God, and then she was looking forward to the future."

Calderon added that Muñoz's journals indicated "she was going through a tough time, but that she loved those kids and couldn't wait to do more in the future with them." 

Iris Carreras

Iris Carreras covers crime for CBSNews.com