(NOTE: Some readers may have inferred from an earlier edition of this story that the federal government keeps a comprehensive record of all prescriptions. The Drug Enforcement Agency says it does track prescriptions of so-called controlled substances -- including some mood-altering medications -- but not all prescriptions made in the United States.)
Cho, born in South Korea, was a legal resident alien of the United States. He was a senior at Virginia Tech, majoring in English.
Sources tell ABC News Cho bought his first gun, a Glock 9 millimeter handgun, on March 13; they say he bought his second, a Walther .22 caliber pistol, within the last week. The serial numbers on both guns had been filed off, they said.
Authorities found the receipt for the 9 millimeter handgun in Cho's backpack. They say the bag also contained two knives and additional ammunition for the two guns.
Legal permanent resident aliens may purchase firearms in the state of Virginia. A resident alien must, however, provide additional identification to prove he or she is a resident of the state.
Sections of chain similar to those used to lock the main doors at Norris Hall, the site of the second shooting that left 31 dead, were also found inside a Virginia Tech dormitory, sources confirmed to ABC News.
President Bush and the first lady addressed the Virginia Tech community at a convocation Tuesday afternoon.
"Yesterday began like any other day. Students woke up and they grabbed their backpacks and they headed for class," Bush said. "And soon the day took a dark turn, with students and faculty barricading themselves in classrooms and dormitories."
"For many of you here today, it was the worst day in your lives," Bush said.
Cho's identity has been confirmed by matching fingerprints on his guns with his immigration records.
"Lab results confirm that one of the two weapons seized in Norris Hall was used in both shootings," Virginia Tech Police Chief Wendell Flinchum said at a press conference Tuesday morning.
Cho, according to law enforcement officials, had entered the country through Detroit with his family in 1992, at the age of eight. He last renewed his green card in 2003. As of yesterday, his home address was listed as Centreville, Va., and the university reported he was living in a campus dormitory, Harper Hall.
Cho's parents live in a townhouse development in Centreville, a suburb of Washington. They own a dry-cleaning shop nearby.
His older sister, Sun-Kyung, graduated from Princeton University in 2004. A source, who asked to be identified as a senior Administration official, said she works for McNeil Technologies, a firm contracted by the State Department to manage reconstruction efforts in Iraq.
Police searched the family home last night. On Tuesday, no one was answering their door.
One neighbor, Marshall Main, describes Cho's parents as quiet and polite. Neither Main nor another neighbor recalled seeing the son in recent years.
Cho graduated from Westfield High School, a Fairfax County public school, in 2003. The school system says two of the dead yesterday at Virginia Tech had graduated from Westfield in 2006; they would have been freshmen when Cho was a senior.
Police say Cho killed two people in West Ambler Johnston Hall, a dormitory near his own, shortly after 7:00 a.m. Monday. Then, two hours later, he opened fire in Norris Hall, a classroom building across campus.