In an online forum, answering a question about state gun laws several years ago, Ms. Harper took a jab at “lame states” that impose limits on keeping loaded firearms in the home, and noted that she had AR-15 and AK-47 semiautomatic rifles, along with a Glock handgun. She also indicated that her son, who lived with her, was well versed in guns, citing him as her source of information on gun laws, saying he “has much knowledge in this field.”

“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 acknowledgement.”

Law enforcement officials have said they recovered 14 firearms and spare ammunition magazines that were purchased legally either by Mr. Harper-Mercer, 26, or an unnamed relative. Mr. Harper-Mercer had six guns with him when he entered a classroom building on Thursday and started firing on a writing class in which he was enrolled; the rest were found in the second-floor apartment he shared with his mother.