Gunman Christopher Sean Harper-Mercer later killed himself as officers arrived, Douglas County Sheriff John Hanlin said Saturday.
Then, two minutes after that, officers told dispatch that Mercer was down, Hanlin said.
Ms Willis, who visited her granddaughter in hospital, said she told her the shooter was asking students about their faith.
The gunman, his background and his motive for the shooting are under investigation, Hanlin said.
The gunman who killed nine people and wounded seven others in Oregon had targeted Christians, the father of one of the victims says.
Her father Stacy told CNN victims had been asked to stand up if they were Christian. The gunman thought Lacey Scroggins was dead as well, stepped over her and shot someone else.
Asked if the shooter had given anything to anyone at the scene, Sheriff Hanlin said he did not know.
The shooter left behind a “multipage, hated-filled” statement in the classroom, according to a tweet from an NBC reporter, citing multiple law enforcement sources who were not identified. The official provided the name on condition of anonymity because he was not authorized to speak publicly.
Authorities have recovered 13 weapons, including six found at the college.
It was unclear how long Mercer had been a student at Umpqua, a small college of about 3,000 students located in Roseburg, a close-knit rural community.
He said: “And they would stand up and he said, “Good, because you’re a Christian, you’re going to see God in just about one second”.
Late on Friday, it emerged that Harper-Mercer was once turned away from a firearms academy by an instructor who recalled finding him “weird” and “a little bit too anxious” for high-level weapons training. In one post, he appeared to urge readers to watch the online footage of Vester Flanagan shooting two former colleagues live on TV in August in Virginia, noting “the more people you kill, the more you’re in the limelight”.
“If you had a couple of the teachers or someone with guns in that room, you would have been a hell of a lot better off”, Trump said on the stump Saturday in Franklin, Tennessee, news reports said.
Mercer’s relatives on Saturday released a statement, saying they were “shocked and deeply saddened by the horrific events”.
“Primarily, at this point what we really want to do is try to gain an understanding so that we can work together with law enforcement agencies all across the country in an attempt to try to prevent future types of situations”.
Nine more people were wounded, three critically, before Harper-Mercer was killed in an exchange of gunfire with two police officers.
Mercer was a student at Umpqua and was enrolled in the class where he opened fire, according to authorities.
Reina Webb, 19, said Harper-Mercer’s mother was friendly and often chatted with neighbours, but her son kept to himself. They ranged in age from 18 to 67-year-old Lawrence Levine who taught the class. They were sons and daughters, spouses and parents.
The gunman carried six guns, body armour and five magazines of bullets with him to campus.
He initially opened fire in an English class, she said.
Several years ago, Mercer moved to Winchester, Oregon, from Torrance, California, with his mother Laurel Harper, a nurse.
At a block of flats where Harper-Mercer and his mother had lived in Southern California, neighbours remembered a quiet and odd young man who rode a red bike everywhere.
Harper-Mercer was in the Army for a month in 2008 before being discharged for failing to meet administrative standards, military records showed.