An FBI agent accused of tampering with drug and gun evidence repeatedly checked heroin out of an evidence room, sometimes using a false signature, and some packages eventually came back damaged, according to court documents made public Tuesday.

Authorities investigating the agent, Matthew Lowry, also discovered that two guns — a Derringer pistol and a Remington shotgun — and marijuana that he seized during a search of a home and car in Maryland were never submitted into evidence.

These new details for the first time describe how Lowry is alleged to have pilfered drugs that were evidence in several criminal cases, forcing federal prosecutors to dismiss charges against at least 28 defendants and notify 150 more that the agent had participated in investigations targeting them.

The documents do not say what Lowry may have done with the drugs after he took them out of the FBI room. In most cases, he eventually took the packages to a Drug Enforcement Administration lab for testing, but authorities now say many came back with weights different from what they were when the packages were taken out. Sometimes, the documents say, packages were heavier, sometimes slightly lighter.

Authorities also said money and drugs went missing from searches and arrests. In one case, according to the court papers, Lowry and other agents reported seizing a large amount of cash from a search conducted in Southern Maryland. A later inventory came up $130,000 short.

U.S. District Judge Emmet G. Sullivan made two documents — the most detailed yet describing Lowry’s alleged misconduct — public by lifting a protective order. The papers are among those prosecutors have provided to lawyers representing 33 defendants in a drug case that Lowry helped investigate but that authorities have said they do not intend to dismiss. One of the defense attorneys, Eduardo Balarezo, argued in court Monday that the court’s previous order against disseminating the material made it difficult for him to conduct an independent review.

Lowry’s attorney, Robert C. Bonsib, did not immediately respond to phone calls or e-mails seeking comment. Lowry has been suspended amid the criminal investigation but has not been charged. Prosecutors said they hope to conclude the investigation by the end of December.

The documents describe numerous instances in which Lowry checked drugs out of the FBI’s Washington field office evidence room but did not get them to the DEA lab until days or, in some cases, months later. In some instances, evidence-control stickers on the packages were found to be damaged or altered once the packages were returned to the FBI, the papers state.

They note that FBI supervisors listed any discrepancies as anomalies, but they do not say whether Lowry was ever questioned or whether authorities were suspicious enough to investigate. An FBI spokesman at the Washington field office declined to comment.

Balarezo, who represents an alleged drug kingpin, Juan Floyd, said in court papers seeking additional information from prosecutors that “if Lowry was able to avoid logging seized evidence, remove evidence for extended periods of time or to otherwise mishandle evidence with impunity, it stands to reason that the strict chain of custody claimed by the government in most cases is a sham.”

Prosecutors argue that Lowry had nominal involvement in the Floyd investigation and that none of the evidence he is accused of tampering with in that case was used against any of the defendants.

The court papers also confirm that Lowry was found Sept. 29 inside his unmarked FBI vehicle near the Navy Yard in Southeast Washington. The documents state that he “appeared to be under the influence of narcotics.”