The police say they made 31 purchases from the five students totaling less than $11,000 for various drugs, including Adderall, cocaine, ecstasy, LSD and marijuana. The authorities said searches of the students’ rooms and other housing at Columbia turned up 4 bottles of LSD, 38 ecstasy capsules, 15 Adderall pills, 2 plastic bags of psychedelic mushrooms, nearly a pound of marijuana and $7,200 in cash.

Illegal drug use is an issue on virtually all university and college campuses in the United States , and Columbia is no different. A number of Columbia students said in interviews that most students who bought drugs did so by visiting dealers in the neighborhoods near the university or through services that delivered to the campus.

The students said that although their five peers were well known among drug buyers at the university — selling mostly to their close friends, freshmen and others in the fraternities — they were part of an illicit trade that was not very extensive on campus. And in the days after the arrests, things seemed unchanged on campus, as students prepared for final exams.

The one difference could be seen on Columbia’s fraternity row on West 114th Street, where university officials suspended activities at three fraternities where most of the drugs were said to have been sold.

Still, the investigation exposed a specialized network, in which the students are said to have sold select drugs out of certain locations, and referred buyers to one another, prosecutors said in court documents.

The Psi Upsilon fraternity house was said to be the place to go for psychedelics: Adam Klein, 20, a member of Columbia’s fencing team, was said to have sold the undercover officer 16 purported LSD tablets from a room there. A student council member, Michael Wymbs, 22, was accused of peddling ecstasy from the fraternity.

The Pi Kappa Alpha fraternity was said to be the center of the Adderall dealing. Christopher Coles, 20, a junior studying anthropology and political science, and Stephan Vincenzo, also 20 and a scholarship student, were accused of being the purveyors.

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Harrison David, a student at the engineering school, was the only one accused of selling cocaine. The authorities said he sold marijuana several times, mostly from Alpha Epsilon Pi and the Intercultural House, along with Mr. Coles and Mr. Vincenzo. One deal was said to be for $5,000. One buyer, in an interview, praised the marijuana variety allegedly offered by Mr. David, characterizing it as “green crack.”

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Most purchases were for relatively small amounts: an ounce of marijuana, a few pills of Adderall or ecstasy. At least two of the students have been said to sell not to earn excess profits, but to support their own drug habits.

Mr. David’s lawyer did not return calls seeking comment.

Crime and safety data that universities and colleges are required to report annually show that the magnitude of the drug problem at Columbia, or at least some degree of enforcement by the university, has significantly increased in recent years. According to Columbia’s figures, the number of students referred to internal disciplinary proceedings for supposedly using drugs went from 8 in 2005 to 121 last year.

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Kevin G. Shollenberger, dean of students for Columbia College and the school of engineering, said that of all the disciplinary referrals made since 2006, roughly half of those students were found to have violated the university’s drug policy and received some kind of sanctions. A vast majority of the cases involved smoking marijuana, he said, though he declined to say what penalties most of the students received. The possibilities include warnings and probation.

Mr. Shollenberger contended the jump in the number of disciplinary proceedings over the years stemmed from changes put into effect at Columbia since 2006, like better training for resident advisers and the creation of the Office of Judicial Affairs and Community Standards.

The measures Columbia has put in place, though, would not have necessarily been effective in rooting out the five students accused of dealing drugs, perhaps explaining why the matter was brought to the attention of the police by an anonymous caller to Crime Stoppers. The call, made toward the end of the last academic year, led to an investigation that initially focused on one student: Harrison David.

Police investigators received anonymous tips about Mr. David, 20, the son of a Massachusetts doctor. Officials said he would unwittingly lead undercover officers to everyone else charged in the indictment — four other Columbia students and three people accused of being his suppliers: Megan Asper, Roberto Lagares and Miron Sarzynski.

Mr. David was entering his third year at the engineering school when the undercover officer gained his confidence. After a few deals, Mr. David got Mr. Sarzynski and the officer connected. One official said Mr. Sarzynski, who came to New York from Poland with his mother and brother about 10 years ago, had sold cocaine to Mr. David.

Mr. Sarzynski conducted his sales from several locations in the East Village, including inside his apartment building and inside the building where his mother, who is listed as a clinical coordinator at Columbia University Medical Center , lives.

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Ms. Asper, 22, who is Mr. Sarzynski’s girlfriend, said several days after being released on bail that Mr. David gave Mr. Sarzynski the phone number of one of the undercover officers so he could call the officer to arrange a cocaine sale.

Ms. Asper said that Mr. Sarzynski had previously been working in partnership with several drug dealers in the West Village, including some who attended New York University , but that they eventually cut him out. Of the charges against her boyfriend, she said: “Miron is small potatoes. I thought the police had bigger fish to fry.”

A grand jury indicted Mr. Sarzynski on a charge of the attempted kidnapping of a rival drug dealer. Prosecutors said his plan was to kidnap the victim at gunpoint, hold him for ransom and torture him by making him ingest LSD.

Ms. Asper added that she first heard Mr. Sarzynski mention Mr. David after the couple had moved to the East Village from Jersey City about seven months ago. Ms. Asper said that on at least one occasion, she and Mr. Sarzynski bought marijuana from Mr. David, but that they found it to be weak and too seedy. She said Mr. David had claimed he was being advanced pounds of marijuana from someone with ties to the Grateful Dead .

Mr. David also introduced the police to Mr. Lagares, the Brooklyn ice cream truck driver. In an interview in jail, Mr. Lagares said he got to know Mr. David over the summer after being introduced through other dealers from “a downtown university.”

Mr. Lagares, arrested on Dec. 5 at his apartment in Bedford-Stuyvesant, said that he sold cocaine to Mr. David and that the most he ever dealt to him at one time was an ounce. For the most part, Mr. Lagares said, he sold Mr. David eighths of an ounce for his own use. He added that they had used cocaine together.

Mr. Sarzynski and Ms. Asper were arrested on Oct. 27. All eight suspects have pleaded not guilty, and only Mr. David, Mr. Lagares and Mr. Sarzynski remain in custody; the others have been released on bail.