"My reporting shows that he did," Roberts said. "My reporting shows that in his Yankee years, he was exhibiting side effects that were noticeable to teammates, that are consistent with the use of ... HGH with a low dose of testosterone."
Roberts' new book, "A-Rod: The Many Lives of Alex Rodriguez," is due out in stores Monday.
In a 45-minute interview with Costas, Roberts addressed criticisms of her reporting, her own motives and her personal beliefs regarding her superstar subject.
The book also details how Rodriguez would tip pitches to opposing middle infielders in blowout games, in the hope that they would return the favor when he was batting.
Those charges are arguably even more inflammatory than his steroid use, the core of which Rodriguez has already revealed. The only contention regarding Rodriguez's use of performance-enhancing substances is precisely when he used them. Rodriguez admitted in February only to doing so while with the Rangers from 2001-03.
In her book, however, Roberts intimates that Rodriguez, facing the pressures of a baseball era shaped in part by known steroid user Jose Canseco, also used performance-enhancing substances at Westminster High School in Miami.
"He was really born into the steroid era," Roberts said. "He was a child of the Canseco era, too. So you have somebody who was born into it, who is in it and is of it, and I think there are so many enablers around him that I do believe he got caught up in a culture that was inescapable for him."
Roberts bases much of that claim on the fact that Rodriguez went from barely being able to bench press 100 pounds during his sophomore year of high school to being able to lift 310 pounds by his junior year.
One of his high school teammates, Major League first baseman Doug Mientkiewicz, told the Los Angeles Times over the weekend that he never saw Rodriguez take performance-enhancing substances, and that it would have been "99.9 percent impossible" for him and his other teammates not to know.
Yet Roberts defended her reporting, the vast majority of which is attributed to unnamed sources.
"I used the same litmus test on all the sources," Roberts said. "The sources were obviously right. They were obviously credible."
The interview took place on the same day that the New York Times reported Major League Baseball investigators are looking into the accuracy of Rodriguez's statements about when he used performancec-enhancing substances. Rodriguez, in an interview with ESPN's Peter Gammons in February, originally admitted only to using over-the-counter supplements that triggered a positive steroid test. But he later admitted to injecting a steroid called "boli" with his cousin, later revealed to be Yuri Sucart.
Roberts questioned that story, claimed that Rodriguez actually received illegal substances from banned trainer Angel Presinal and latched onto the league's reported investigation as vindication for her skepticism.
"You don't investigate somebody if you just think they told you the truth," Roberts said.
Yet given the league's lack of subpoena power and the lack of hard evidence, Roberts said she did not expect the investigation to bear any punitive repercussions.
"That has been the thread throughout the steroid era," Roberts said. "It's so hard, and so difficult to make any anecdotes, to make allegations, to make even the visual evidence, to make the he-said, she-said stick, because the testing has not been there to back up the investigation."
Roberts also spent a portion of the interview defending the existence of her book, which critics have derided as an unnecessary attack on Rodriguez's character. Yankees manager Joe Girardi vocally denounced the book prior to Sunday's scheduled game that was rained out against the Angels, saying he does not "understand why someone would write a book like that, anyway."
Roberts said that she began her work under the assumption that Rodriguez was clean, and that the book grew more negative due to the nature of what her reporting uncovered.
"To say that he's not worthy of the book would be to say that he is not worthy of inspection, when he is one of the greatest players to play the game," Roberts said. "He's the richest player to play the game. He's somebody who's certainly been polarizing."
Rodriguez opted out of the final three years of a record $252 million contract during the 2007 World Series, only to re-sign with the Yankees for 10 years and $275 million.
The money, according to Roberts, hints at his psychological insecurities, which she says stem from a father that abandoned him and a "Joe DiMaggio" complex that made him yearn for the public spotlight. Much of the book details Rodriguez's personal life, particularly his relationship with pop icon Madonna and his resulting divorce.
Roberts said that she sees her book, particularly the early public outcry toward it, as an opportunity for Rodriguez to again endear himself to a once-adoring public. In her reporting, Roberts said, she found that many of her sources were actually sympathetic to the plight of the three-time AL MVP, statistically one of the game's greatest players of all time.
"A lot of them were very concerned about Alex Rodriguez," Roberts said. "A lot of them think, here's a guy who has such potential to do amazing things in baseball. He was so great he didn't need the embellishment. He didn't need the enhancement. He was a terrific player without becoming a tall tale."