BLACKSBURG, Va. — The lone gunman behind the worst mass shooting in American history harbored a deadly grudge against his university and was obsessed with a female student, police disclosed yesterday.

In a note discovered by the FBI in his dormitory room, Cho Seunghui, 23, a South Korean student, railed against “rich kids,” “debauchery,” and “deceitful charlatans” at Virginia Tech, where he shot dead 28 students and four lecturers before turning his gun on himself. All of the victims were shot three times.

The note, which ran to several pages, also stated, “You caused me to do this,” which was thought to have been a reference to 18-year-old Emily Hilscher, his first victim.

She was killed in her room at the West Ambler Johnston Hall accommodation block along with a dormitory warden, Ryan Clark, 22.

Cho came to America at the age of 8 with his parents, who run a dry cleaning business in suburban Virginia. A university official said the English literature student was “troubled” and his essays had been so disturbing that he was referred to the campus counseling service. It was feared that he suffered from depression.

“There was some concern about him,” said Carolyn Rude, head of the English department. “Sometimes, in creative writing, people reveal things, and you never know if it’s creative or if they are describing things.”

A play he wrote last year as part of a writing class gave some clue to his thinking. Entitled “Richard McBeef,” it featured a 13-year-old boy who accuses his stepfather of pedophilia and murdering his father. The teenager talks of killing the older man, and, at one point, the child’s mother brandishes a chainsaw at the stepfather. The play ends with the man striking the child “a deadly blow.”

Police said Cho had used two pistols purchased legally to carry out the massacre. The first, a Glock 9mm, was bought for $570 on March 13, and the second, a Walther P–22, just three days before the shooting spree.

President Bush joined students, many in tears and wearing their VT college sweatshirts, at a remembrance service at the university. He said the victims were “in the wrong place at the wrong time.”

“It’s impossible to make sense of such violence and suffering,” said the president. “Those whose lives were taken did nothing to deserve their fate. They leave behind grieving families, grieving classmates, and a grieving nation.” He encouraged students and families to find strength in prayer.

“People you have never met are praying for you, and praying for your friends fallen and injured,” he said. “There is real power in these prayers and in times like this we find comfort in the greatness of a loving God.”

Some students continued to ask why police and university authorities did not clear the campus after Cho shot Hilscher and Clark at about 7:15 a.m.

It was more than two hours later when he entered the Norris Hall teaching building, chained the main doors behind him, and killed another 30 victims. Cho burst into at least four rooms and shot anyone in sight.

Students received their first email warning at 9:26, minutes before the carnage began.

“The university has blood on their hands because of their lack of action after the first incident,” said Billy Bason, 18, who has a room on the seventh floor of West Ambler Johnston Hall.

Richard Steger, the university president, said the university was trying to notify students who were already on campus, not those who were commuting in.

“We warned the students who we thought were immediately impacted. We felt that confining them to the classroom was how to keep them safest,” he said.

He said investigators did not know there was a gunman loose on campus in the interval between the two shootings because the first could have been a murdersuicide.

Witnesses had told police that the first murders involved a “domestic dispute,” leading police to pursue a man believed to have been Hilscher’s boyfriend.

They were still questioning him when the second spate of shootings happened half a mile away at Norris Hall.

Detectives were working on the theory yesterday that Cho had become obsessed with Hilscher, a veterinary science student. She became his first victim shortly after he woke her up.

Police were unsure whether Clark, who lived on the same floor, had run to help after he heard a commotion or whether he had been in Hilscher’s room.

There were suggestions that Cho believed he had been jilted by Hilscher, but police said they had no evidence that the two had been in a romantic relationship.

A student who shared facilities with him never saw him with a girl — or any friends. Karan Grewal, 21, painted a picture of a loner who ate his meals alone in the dining hall and shunned any attempts at friendship.

Even the student with whom Cho shared a room talked of him as a stranger.

“He was my roommate,” said Joe Aust, 19, an engineering student.” I didn’t know him that well, though.”