The campus newspaper, The Daily Princetonian, reported that security officials had entered Calvo's office and demanded he return his keys, informing him of his dismissal.
Aponte said he'd spoken with another close friend -- Ana Belen Martin-Sevillano, a professor in Canada -- who said Calvo was "very distressed" just a week before his death but that "he was still going to fight it. He didn't think it was fair -- no gross misconduct was recorded. That's why we all believe somebody had a personal problem with him.
"He was getting more and more nervous, according to Ana," Aponte said. "And he wasn't eating very much."
ABCNews.com spoke to Martin-Sevillano, who said she was too upset to talk now.
Another friend, who got worried after he received a "goodbye letter" from Calvo, according to Aponte, found Calvo's body in his Chelsea apartment April 12.
"Antonio was an absolutely remarkable man and friend," Princeton student Philip Rothaus, who graduates this year, told ABCNews.com. "Funny doesn't begin to cover it. We used to send each other hilarious and totally bizarre YouTube videos while G-chatting at 2 a.m. Even when he wasn't acting as my professor or adviser, he was amazingly generous and helpful, offering advice on papers, translations and other academic issues. His knowledge was unbelievably broad. He gave so freely of himself."
"What's also becoming apparent is that his productivity was superhuman," he said. "We truly loved him and will miss him deeply."
Rothaus had written to the university, saying that Calvo's contract may have been canceled "against the wishes of [his] department," according to an open letter obtained by IvyGate.
"Antonio's dear friends, his colleagues in the Spanish and Portuguese department, have been forbidden from speaking about this to anyone," he wrote. "I am, thankfully, not subject to the same constraints, and, at this point, am angry enough not to care.
"Herein lies the greatest mystery: They must have had some reason -- otherwise nothing makes sense -- but they continue to suppress it," he wrote. "There were two weeks left of classes that he was teaching, and of school for students he was advising -- was he dangerous? If so, where is the evidence? And why won't they tell even the department itself?"
When ABCNews.com asked for more details about the university's handling of the incident, which was reported by the Princetonian as a "cover-up," Rothaus responded, "I think "cover-up" is an inflammatory and misleading term. Please respect my privacy."
Molly Bagshaw, a former student and advisee told the The Princetonian, "Antonio had a huge impact on me, in terms of both my academic career and love of the Spanish language."
"If it weren't for his advice and counseling, I probably wouldn't be pursuing a certificate in the department or going to Peru this summer, two things I am extremely excited about," she said.
Calvo, who was hired in 2000, also taught linguistics, poetics and the arts of the Harlem Renaissance. He was also passionate about the music of Greg Ogsby and Lupe Fiasco, photography and hiking.
Aponte vowed to find out why his friend and colleague had resorted to suicide over what seemed like campus politics, and he had no explanation for the violent method used. "It's awful," he said. "It's awful."
This was the second high-profile suicide at Princeton in three months. In January, 27-year-old Bill Zeller, a promising graduate student who had been haunted by childhood abuse, died after hanging himself.