CNN) -- A 20-year-old man walked into a plasma center in Virginia and asked to use a restroom.
Then he went on a stabbing rampage with a weapon similar to a machete, wounding four people, police said.
The suspect hid the weapon as he entered the Octapharma Plasma center in Petersburg on Thursday, according to police Capt. Emanuel Chambliss.
He described the weapon as similar to a machete, saying police are not sure what it is.
The suspect waited for police to arrive
When the stabbings started, some people fled through the back door and called the police.
After the attack, the suspect did not try to run.
"He was sitting in a chair waiting for us," Chambliss said.
He was arrested at the scene without incident and police don't know of a motive for the attack.
The suspect, who has not been identified, is a previous donor at the clinic, police said. The investigation is still active.
Two victims are in serious condition
Three women and one man were transported to a local hospital, and two were in serious condition after surgery Thursday night.
Two victims were treated and released.
Petersburg is 25 miles from Richmond. It has a population of about 33,000.