Although the psychologists and psychiatrists we talked to did not evaluate or treat Lee, they said their opinion was based on media reports and Lee's written missive.
"He clearly had paranoid delusions or perceptions, and he was clearly psychotic," said Dr. Jeffrey Lieberman, chief of psychiatry at Columbia University Medical Center in New York.
"This is a person that is delusional and has a thought disorder of some sort, and one kind of thought disorder is paranoid schizophrenia," said George Pratt, past chairman of psychology at Scripps Memorial Hospital in La Jolla, Calif.
Paranoid schizophrenia is a serious mental illness characterized by delusions and auditory hallucinations, as well as paranoid thinking. It affects about 1 percent of the population and can be treated with medication and other therapeutic tools.
Mental health experts believe Lee was likely plagued by personal problems.
"He is most likely a loner with disrupted family relations and a very difficult work history," said Stevan Hobfoll, professor and chairman of the department of behavioral sciences at Rush University Medical Center in Chicago.
"He likely had a great deal of trauma and rejection in his life," said Pratt.
Lee's sister, Tuonglan Lee, told ABCNews.com her brother was estranged from the family since 2002.
She also said he was arrested and jailed for human smuggling in 2003. Of that time in his life, James Lee told the judge who sentenced him that he was choked and robbed and "subjected to more of life's brutality."
Experts believe a brain dysfunction lays the groundwork for schizophrenia to express itself later in life, and that groundwork, they said, combined with anger, hostility and self-hatred can cause paranoid schizophrenics to act out in dangerous and unpredictable ways.
They find it telling that Lee, a radical environmentalist, targeted the Discovery Channel. Before Wednesday's events, he'd been arrested for protesting outside the network's headquarters in 2008 and had been demanding a boycott of Discovery's programs since then.
"It becomes a whole working delusion, because if you're an environmentalist, the Discovery Channel is a bizarre target," said Hobfoll.
"The idea of going to a channel that's dedicated to educational programming is distorted," said Hobfoll.
"The way he writes his message about making the world a better place and the way he goes about promoting it are at odds," said Dr. Ken Robbins, clinical professor of psychiatry at the University of Wisconsin-Madison.
If Lee was a paranoid schizophrenia, experts said it's unlikely he was psychopathic.
"He might genuinely feel very deep emotions," said Hobfoll. Psychopaths, on the other hand, lack a conscience and empathy for others.
Schizophrenia is treatable, and experts said medication is effective in controlling irrational thoughts. They also believe Lee was not taking medication or under any treatment for his condition at all.
Police eventually shot and killed Lee. His brother, Aaron Lee, told ABC News that that was probably exactly what James wanted.
Despite that, Lieberman said there was no way to predict the standoff would end the way it did.