Speaking to reporters after a court hearing in which a judge ordered the suspect, Aaron Ybarra, 26, held without bail on suspicion of first-degree murder and assault, defense attorney Ramona Brandes also said her client had been involuntarily committed in the past because of mental illness.
She added that Ybarra was sorry for the victims' pain.
The probable-cause statement filed in court by prosecutors said Ybarra confessed to police detectives that he was the gunman in Thursday's incident, that he had been planning a mass shooting and wanted to kill as many people as possible before taking his own life.
Local media reports citing unidentified police sources have said Ybarra, who is not a student at the college, was fascinated with mass shootings at other schools, including the 1999 Columbine High School massacre in Colorado.
Police offered no public explanation for why the suspect might have singled out Seattle Pacific University, a Methodist liberal arts college of some 4,000 students.
Ybarra is accused of walking into a building on the campus armed with a shotgun and opening fire on three people before pausing to reload his weapon.
At that point, police said, a student building monitor doused the gunman with pepper spray and tackled him. Several bystanders jumped in to help, seizing his gun. He was arrested minutes later by police, who said the suspect also was carrying a hunting knife and at least 50 rounds of shotgun ammunition.
SUSPECT WAS COMMITTED TWICE
In a statement read to reporters on Friday, Brandes said: "Mr. Ybarra suffers from significant and long-standing mental health issues, including delusions, that were in play during yesterday's tragedy.
"I do not know at this point in time why Mr. Ybarra's illness brought him to Seattle Pacific University, but I can tell you that he recognizes the suffering of the victims and their families. He is sorry for their pain," she said.
Brandes said she did not know the circumstances of his prior involuntary commitment. Police in Mountlake Terrace, a suburb south of Seattle, said on Friday that Ybarra had been detained and committed to mental health facilities twice after erratic behavior in 2010 and 2012, without giving any details.
Citing unidentified police sources, local KIRO-TV said Ybarra had visited the Columbine high school in Colorado where two students killed a teacher and 12 classmates before taking their own lives in 1999.
A 19-year-old SPU freshman who was shot during the rampage died a short time later. A 20-year-old woman remained hospitalized on Friday in serious condition in the intensive care unit, although she was conscious and breathing on her own.
A 24-year-old man was in satisfactory condition with pellet wounds. A 22-year-old man was treated for minor injuries suffered in the scuffle with the suspect, police said.
The bloodshed in Seattle marked the latest in a series of mass shootings at schools and other public places across the United States in recent years that have renewed a national debate over gun safety and mental illness.
Two weeks ago, a 22-year-old man stabbed three people to death and fatally shot three others before taking his own life in a rampage across a college town near the University of California at Santa Barbara.