Mia Ylonen, a radio journalist, told Al Jazeera from Helsinki: "The man entered the school via the basement wearing a balaclava and carrying a briefcase.
"He then shot people in the head at close range."
Jukka Forsberg, a school caretaker, told Finnish broadcaster YLE: "Within a short space of time I heard several dozen rounds of shots ... it was an automatic pistol.
"He also shot towards me, did not say anything and once the bullets started to whizz by I started running for my life."
Anne Holmlund, Finland's interior minister, said many of the students at the post-secondary school, which teaches catering and tourism studies, are about 20 years old.
Saari had been questioned by police a day before the attacks after he posted footage of himself on the Youtube video sharing website carrying out target practice with guns.
Holmlund said: "Police action will be examined in more detail later. The gunman had a temporary permit for a .22 calibre pistol, and he had received it in August 2008. It was his first gun."
Police did not take away his weapon after the interview and Homlund said that Saari had been released because there was no legal reason to hold him.
Matti Vanhanen, Finland's prime minister, said: "We have experienced a tragic day."
The Finnish government said grief counsellors were on site and giving support to students, teachers and relatives.
Finland has the third-highest level of gun ownership in the world, behind the United States and Yemen, a survey carried out last year by the Small Arms Survey of the Graduate Institute of International Studies in Geneva said.
Speaking to Al Jazeera, Laura Lodenius, part of Finland's anti-gun lobby, said: "This is a sad day. We are also angry. We have to have stricter laws on where weapons are kept and who are given licences.
The Kauhajoki vocational school where the shooting took place [AFP]
"We have of course a tradition of hunting in Finland and that that is the reason that people think we have so many guns in Finland.
"I think now that people are maybe starting to open up their eyes and realising that today's gun owning is not that of 50 or 100 years ago and that it's really a problem in today's world."
However, Alexander Stubb, Finland's foreign minister, said that the country's gun laws were already very strict.
""We do have a high number of arms - 1.6 million to be exact, and they're based on 650 licenses and for each license you actually need to be interviewed by the police, so we're not talking about free guns per-se in Finland," he said.
"But I'm sure that we will now in Finland start a discussion also ... the difference between what could be called hunting rifles and just basic traditional handguns."
Tarja Halonen, the Finnish president, acknowledged that the "shocking" news would start a national dialogue on gun ownership.
"We have to have very serious discussions and studies on what to do," she said.
The attack came 10 months after eight students were killed in a gun attack at a school north of Helsinki.
After that shooting, the Finnish government took some steps to toughen gun regulations.