The shooter, Steven Kazmierczak, entered a lecture hall and opened fire before turning the gun on himself.
Last April, a gunman killed 33 people on the Virginia Tech campus.
The Virginia Tech students wanted to give the NIU community a shoulder to lean on.
"We received an overwhelming amount of support from different colleges and all around the world last April. And we think we need to answer that call again when someone else is in trouble," said Paul Masur, Virginia Tech student.
The Virginia Tech students also brought a banner signed by their classmates, sending well wishes and messages of support to NIU's students.