Two handguns, a 9 mm one and a 22-calibre one, were used by the South Korean student when he went on a shooting spree at the Virginia Tech university killing 32 people before turning the gun on himself.
Investigators said Cho Seung-Hui, 23, had bought the weapons legally. The victims of the Monday massacre included a professor and a student from India.
A March receipt for the Glock 9 mm pistol was found in the killer's backpack. As a permanent legal resident of the US, Cho was eligible to buy the gun because he had no prior felony criminal charges against him.
Though no suicide note has been found, investigators found a note in his dormitory in which he spoke out against "rich kids," "debauchery" and "deceitful charlatans" on campus.
Cho had been taking medication for depression and was reportedly becoming increasingly erratic and violent.
The sprawling university, which attracts a large number of students from Asia, is located in the town of Blacksburg in Virginia.
Ballistic tests on the evidence seized from the Norris Hall and the West Ambler Johnston Residence Hall were conducted at the US Bureau of Alcohol, Tobacco, Firearms and Explosives lab in Maryland. Lab results confirmed that one of the two weapons seized in Norris Hall was used in both shootings.
"At this time, the evidence does not conclusively identify Cho Seung-Hui as the gunman at both locations," said W Steven Flaherty, superintendent of the Virginia State Police.
"With this newfound ballistics evidence, we are now able to proceed to the next level of this complex investigation," he said.
Cho had arrived in the US in 1992 with his family and was raised in the Washington DC area.
Cho suspected his Taiwanese girlfriend of seeing another man and had a row with her in the West Ambler Johnston Residence Hall, a co-ed dormitory in the sprawling campus of the university Monday morning. When a resident advisor came to resolve the problem between the two students, Cho shot him. He then shot the girl.
University police, who came to West Ambler in response to an emergency call, told inmates of the dormitory to stay inside and started investigations.
Even as they started homicide investigations along with Blacksburg police, Cho went to the Norris Hall, which houses faculty offices, classrooms and laboratories and closed all exit points from that building. He then went on shooting people inside the building.
According to one witness, "he shot every person thrice" with a smile on his face. G.V. Loganathan, the 51-year-old Indian origin professor of civil and environmental engineering, was taking a class when Cho shot him in his head.
According to reports, also killed was Minal Panchal, a 26-year-old female Indian student who was attending Loganathan's class.
Meanwhile, Indian embassy officials have reached the university to help Loganathan's family.
"Members from the Indian embassy have already visited us and provided the necessary support and condolence to the families of the victims," a posting on the website of the Indian Students Association (ISA) of the university stated. The ISA has some 500 members.
The ISA, while mourning Loganathan's death, has not yet mentioned Panchal's name.
Virginia Tech has cancelled all classes for the rest of the week.
Norris Hall, where Loganathan and Panchal were among those killed, has been closed for the rest of the semester.
The university will hold a public candlelight vigil Wednesday evening at the Old Town Market Square, in Alexandria, Virginia, around 10 km south of downtown Washington DC to honour the victims.
According to the university, the names of the 32 deceased students and faculty will be released once all victims are positively identified and next-of-kin notified.