Rather, confidential informants - often drug dealers themselves who work for cash or leniency - did most of the transactions.
Philadelphia's practices are similar to those of other big-city forces, according to several studies.
Deputy Police Commissioner William C. Blackburn, who oversees the Narcotics Unit as the head of major operations, said there was a reason for the Philadelphia antidrug force's "heavy reliance" on informants.
"Informants give us an opportunity to get into an area where an undercover police officer wouldn't be able to go," he said in an interview last month.
Cujdik's relationship with one informant has shown the weakness of the system.
Investigators began to pursue Cujdik (pronounced CHUH-dik) last year after his most productive informant, Ventura Martinez, alleged that the officer had repeatedly fabricated evidence to obtain warrants. In January, Cujdik was put on desk duty and ordered to turn in his service weapon.
According to sources, FBI and Internal Affairs investigators are now looking at Cujdik's work with at least four informants, and suspicion has spread to other members of his squad. Philadelphia's public defender has sought to overturn 53 convictions allegedly based on tainted searches.
According to Cujdik's affidavits, the officer cited Martinez - code-named "Confidential Informant 103" - in 43 percent of his 186 search warrants in the last three years. Martinez provided tips to Cujdik and made controlled drug buys from the dealers he informed on. But Cujdik also trusted Martinez sufficiently that he asked him to make drug buys from dealers he had never met.
Often described as a "necessary evil" in narcotics trajectories, informants provide investigators with access and security. They allow undercover officers to avoid dangerous situations with suspected drug dealers. And a productive informant can generate far more cases than officers would if they had to infiltrate the drug world themselves.
For all the work they do, informants don't cost much.
Blackburn said the Philadelphia force maintained about 200 active informants, who were paid a total of $125,000 last year - slightly more than Cujdik's overtime-enhanced salary of $111,800.
Civil libertarians and defense attorneys say informants can encourage police to take shortcuts. Even with safeguards, there is little external oversight to detect misconduct. Judges rarely question whether the informants actually did what the officers say they did.
Even if Martinez is lying - Cujdik's defenders say the informant is making it all up - the doubt he has cast is likely to take a substantial toll. About 500 cases were thrown out after a 1995 evidence-faking scandal engulfed the 39th District, and the city paid more than $4 million to settle lawsuits.
A bigger casualty may be faith in the force at a time when Police Commissioner Charles H. Ramsey has made public trust a cornerstone of his crime-fighting effort.
"The investigation is probably going to grow in scope," Ramsey said. "We will let the chips fall where they may."
A longtime police tool
In the American system of justice, where defendants are often encouraged to bargain for leniency, informants have long been a powerful police tool.
Since the war on drugs escalated in the 1980s, federal courts have relaxed restrictions on confidential informants, or CIs.
A University of Arizona study of federal search warrants in Atlanta documented a "dramatic rise" in the use of informants in the 1980s. The National Law Journal showed the number of federal search warrants that relied exclusively on an unidentified informant increased from 24 percent to 71 percent between 1980 and 1993 in four cities.
With a few exceptions, the identities of confidential informants remain secret.
"You have no opportunity to call the CIs on the stand to examine them," said Guy R. Sciolla, a Philadelphia criminal-defense lawyer, who says it is increasingly rare for undercover officers to make drug buys themselves.
Legal experts say the informality and secrecy of the police's relationship with informants invites abuse.
"The dangers of abuse of false information and of manipulation of the system is widespread, and this has been going on for decades," said Alexandra Natapoff, an expert on informants at Loyola Law School in Los Angeles.
In Florida, the state legislature is considering tightening supervision of informants after Tallahassee police pressured a 23-year-old college student to go undercover. She was murdered, and a grand jury later said she was in "way over her head."
In Georgia, three former Atlanta police officers received jail sentences this year for their roles in the death of Kathryn Johnson, 92, whom police shot during a 2006 drug raid based on bogus evidence attributed to an informant.
After Johnson's killing, the officers planted marijuana in her basement to create the appearance of a drug house and asked a confidential informant to concoct a story to support their account. The informant went public instead.
The Atlanta debacle prompted a 2007 congressional hearing on informants.
In Philadelphia, the police adopted "Directive 15" in 1986 that outlines the "professional manner" in which officers must conduct relations with confidential informants. Everything is supposed to be closely supervised.
The policy spells out how supervisors must approve all informants and be told of any contact with an informant. Officers are to maintain "professional objectivity" with informants, including no exchanges of gifts or gratuities.
Cujdik, 34, is accused of crossing the line during his seven-year partnership with Martinez by renting a house to the informant's girlfriend. The arrangement came to light in October when a lawyer exposed Martinez during a trial, and Cujdik started eviction proceedings.
Law enforcement experts say that officers must guard against efforts by informants to get too friendly. But it's not uncommon for an officer to become dependent upon an informant to generate arrests and seizures that win the officer commendations.
"For these officers, the relationship with their source of information is a critical part of the way they conduct business on the streets," said Gregory A. Paw, a former federal narcotics prosecutor in Philadelphia.
"It creates a world of secrecy," he said, "and it's difficult to operate in secrecy in the justice system."
Commanders, recognizing the temptations that officers face, say narcotics investigators are the most closely supervised on the force. Saying he wanted to tighten control, Ramsey this month broke up two narcotics squads that were supervised by corporals - including Cujdik's old platoon - and put them under the control of sergeants.
Blackburn also said the department began a review of the 23-year-old informant policy last year with an eye toward eliminating weaknesses. He said the revisions might include more specific rules on officers' contact with informants.
But Blackburn said that even a stronger policy could not prevent an officer who intends to circumvent the regulations from doing so.
"If somebody wants to break the rules, you can't policy that out," he said.
The department is also seeking to change the contract with police to make it easier to transfer officers out of special units like narcotics.
Some experts have suggested that officers should regularly rotate in order to reduce complacency and corruption.
But Blackburn, who has spent much of his 27-year career in narcotics, said he believed that undercover operatives had special skills and not everyone should be transferred out.
"Narcotics is a different animal," he said. ". . . It requires real good officers. It's a delicate assignment. A lot of people, it's just not for them."
Laying the groundwork
In the best of circumstances, a CI conducts several introductory buys with a suspected drug dealer before an undercover officer enters the picture, Blackburn said.
"Maybe the first or second buys will be made by a confidential informant," he said. "But at some time, we would like to introduce one of our undercover operatives into the drug operation."
In practice, Jeffrey Cujdik appeared to make no drug buys himself during the last three years, according to hundreds of affidavits reviewed by The Inquirer. His warrants frequently were granted on the basis of one controlled buy by a confidential informant.
Cujdik's brother, Richard, also an undercover narcotics officer who this month was put on desk duty as part of the investigation, amassed a similar record during the last three years, records show.
Though Richard Cujdik made several undercover buys of marijuana himself in 2006, he was more dependent upon a single informant than his brother. Sixty-two percent of the 113 warrants he received cited the same confidential informant - "N-142" - as the source.
Despite the work done by informants, the head of the city's police union said many officers regard them with contempt.
"Any confidential informant is bad per se," said John J. McNesby, president of Lodge 5 of the Fraternal Order of Police, who worked 10 years as an undercover officer. "They're not the most credible people."
Blackburn said he would rather put a confidential informant at risk than a police officer.
"I'm not saying that we want to put the community or the confidential informant at any risk factor, or undue risk. We want to minimize the risk there, too. But I'm looking at the welfare of the police officer first."
Contact staff writer Andrew Maykuth at 215-854-2947 or firstname.lastname@example.org.
Contributing to this article were Inquirer staff writers Dylan Purcell, Gail Shister, Zoe Tillman, and Martha Woodall.