"MS13 has rules, you understand?," says Ismael Cisneros, who is serving life in prison for Brenda's murder. "And when you break the rules, we all have to decide if you deserve to die."
Talking to police, Cisneros says, was breaking one of the gangs rules.
"I considered her a friend. She was a gang member. She was a home boy. That's what we called one another in the gang," he says.
And as a homeboy, Brenda knew the rules. She had initially gone into the federal witness protection program, but after she got pregnant, she was so isolated that she went back to Virginia. She was convinced that no one knew she was working with police. But Det. Freeman, who investigated Brenda's death, said the gang was spying on its own members.
"Mr. Cisneros was recorded and he's actually talking about conducting a private investigation into whether or not she's cooperating with law enforcement," says Freeman.
Not only does MS-13 conduct investigations of its own, but like a corporate organization, most cliques have regular meetings where they discuss recruiting, money and murder – what they call a "greenlight."
According to witnesses, the gang took a unanimous vote in a hotel that Brenda should be assassinated. The next morning, she was lured away on a fishing trip with her new boyfriend, Oscar Grande, and her friend, Ismael Cisneros.
A former MS-13 member who is now in jail on an ammunition possession charge and asked 60 Minutes not to use his name, went with them.
"I was facing the river. You know, I was watching, I was enjoying the view. Was summertime. It was nice place. And they was behind me fixing the fishing pole. And I turn my face. I see for couple seconds that she was get stabbing. And I freak out and I run away," he recalled.
Asked to confirm if he saw the stabbing of Brenda Paz, he answered "Yes."
She was stabbed by her boyfriend Oscar Grande and Ismael Cisneros, who later confessed. He said she had called out "Why?" "Because you're a rat" she was told. They stabbed her approximately 13 times.
"One is not born an assassin or born practicing where you have to stab someone to kill a person on the first try. I would have wanted for her not to have suffered, you understand? From the point where I was, it was painful for me. Everything that happened. And it keeps being painful even up to right now," says Cisneros.
But Brenda suffered the ultimate.
"We have all suffered, sir. Brenda suffered, we all suffered. Not only Brenda," Cisneros told 60 Minutes.
The three men were tried, along with another gang member, for Brenda's murder. Cisneros and Grande were convicted and sentenced to life in prison. The third man at the scene, whom prosecutors alleged held a rope around her neck, took the stand in his own defense and was acquitted.
60 Minutes talked to one of Brenda's closest friends in MS-13, who was disguised extensively because of the risk to her life.
What's the most important thing for people to know about the gang?
"They're dangerous. They're very dangerous," she explained.
She was so outraged by her friend's murder that she decided to do what Brenda had done: break ranks and testify.
She says she didn't care that she might be possibly marked for death for testifying. "There's a baby involved. I mean, you have to be sick to actually hold a rope around somebody's neck, hear her screaming and fighting, being tortured and stabbed multiple times in the stomach, legs, and just dying," she said.
She said that Brenda's murder was a sign of how MS-13 was starting to transform itself into a more elaborate criminal organization.
"It's everywhere. There's MS in New York. There's MS in Florida. I mean, they're everywhere. Texas, Virginia, Minnesota," she said.
She says the gang is in the drug trafficking business, and more. "They traffic guns, drugs, whatever. Stolen cars."
And she says the worst thing she saw was rape. "Young girls being taken into the bathroom and one would go in and one would come out. Another one would go in, come out. And you just hear screaming and sometimes just get quiet."
And, authorities say, cliques send representatives to regional meetings, as seen on a police surveillance tape, where they dole out punishment for violating rules and they talk about expansion. And that's increasingly worrying to the head of the investigative division at the FBI, Assistant Director Chris Swecker.
The FBI has just formed a special task force to deal with a big gang for the first time. Why?
"Well, with respect to MS-13, we don't look at them as a typical gang. MS-13 has two characteristics that give us great concern and have drawn our attention. One is that they are extremely violent, and they're proliferating around the country. Two is they're an international criminal organization. They're not confined in the United States. You can find them in five countries. And now even in Europe," says Swecker.