The Oregon shooter's mother wrote online about her son and her gun collection
The New York Times has uncovered a Yahoo Answers profile believed to belong to Ms. Mercer.
Under the username Tweetybird, Mercer - a registered nurse - spent "hours" on the site answering medical questions from strangers and talking about her hopes for her son, writes Mike McIntire of the Times.
On October 1, her 26-year old son took his own life along with the lives of nine other people during a shooting rampage at Umpqua Community College in Roseburg, Oregon, where he was a student.
Answering a question on a forum about autism, Ms. Mercer once wrote that both she and her son had Asperger's syndrome, an autism spectrum disorder. In another post, she shared that her son was a "headbanger" when he was younger and was initially misdiagnosed with attention deficit disorder.
In the forums, she attempted to encourage other parents who have kids with disabilities by explaining how her son had worked to overcome his issues.
"He's no babbling idiot nor is his life worthless," Ms. Harper wrote. "He's very intelligent and is working on a career in filmmaking. My 18 years worth of experience with and knowledge about Asperger's syndrome is paying off."
She also recommended that parents read to their autistic children and use many expressive gestures. In 2009, she wrote that she used to read her son a book by Donald Trump - the real estate mogul who is the current frontrunner for the GOP presidential nomination.
"Fact: Before my son was even born, I was reading out loud to him from Donald Trump's 'The Art of the Deal,'" she wrote. "And as for the 'gesture effect,' I was practically a mime. And now my son invests in the stock market along with me, turns a profit and is working on a degree in finance. His language and reading skills are phenomenal. I tell you this because it's not too late for you to start helping your daughter."
Ms. Harper never mentioned her son by name, the Times notes.
On guns, Ms. Harper wrote in a forum on state gun laws several years ago that she owned AR-15 and AK 47 semiautomatic rifles as well as a Glock handgun.
"I keep two full mags in my Glock case. And the ARs & AKs all have loaded mags," Ms. Harper wrote. "No one will be 'dropping' by my house uninvited without acknowledgment."
She disagreed with states that impose limits on keeping loaded firearms in a home, calling them "lame states," and said that her son was well versed in guns, saying he "has much knowledge in this field."
Ms. Mercer said that she would sometimes go shooting with her son. She also talked about the importance of firearm safety, and expressed distaste with unprepared gun owners.