A student who went on a rampage with a baseball bat in a university classroom might have snapped because of failing grades.
Four classmates were beaten as they leapt to the defence of the lecturer, who appeared to be the target.
A blood-stained cloth is one remnant of a random attack during a lecture at Australian National University in Canberra.
The alleged attacker is an 18-year-old student who launched a baseball bat at a female lecturer.
Witnesses say other students intervened and tried to restrain the man.
"I believe one of the students has actually taken the baseball bat from the student," Detective Superintendent Ben Cartwright said.
During the struggle a female student was struck in the head, requiring stitches.
Another student suffered a broken arm, while a third student has fractures. The lecturer was also injured in the attack.
"One of the people that we know saw all these kids running out with people, like, having blood gushing down their face," one student said.
As students ran for cover, one male student managed to subdue the attacker until campus security arrived.
"It was incredibly brave on behalf of those students to protect the lecturer," Det Supt Cartwright said.
Fellow students have told Seven News the young man was normally "quiet" and "reserved" in class.
While they are struggling to understand why this attack happened, sources in ACT Police have told Seven News they are examining mental health issues as a possibility.
It has also been suggested that the student was struggling with his studies and possibly took his resentment out on his teacher.
Deputy Vice Chancellor Marnie Hughes-Wellington thanked the students who had intervened.
"I just really want to commend those students, and everybody else that's assisted this morning in a deeply shocking incident," she said.
The alleged attacker is being questioned by ACT Police but is yet to be charged.
Of the four people injured, two have been released from hospital.
The male student who restrained the alleged attacker and the female lecturer remain in hospital.