The Secret Service is closing its investigation into who may have brought a plastic bag of cocaine into the White House this month after lab results were inconclusive about possible suspects, according to two people briefed on the probe.

The Secret Service sent the bag that had contained the powder to an FBI lab to look for traces of DNA and fingerprints, but neither form of testing yielded definitive results, the agency said. Nor was any surveillance video found that provided any investigative leads, officials added.

The Secret Service identified roughly 500 people who had entered the general area of the West Wing during the time before the July Fourth holiday and could have brought the cocaine inside the White House, according to the people.

The likelihood of finding conclusive forensic evidence on a small plastic bag was always considered small, according to several law enforcement officials familiar with the probe, who like others spoke on the condition of anonymity to discuss sensitive matters. But the rapid closure of the probe after 10 days - and the seeming lack of pursuit of any other investigative steps to find out how someone sneaked cocaine into the White House - prompted criticism from Republican lawmakers.

"The White House is supposed to be the most secure residence in the world, but today Secret Service officials failed to answer basic questions or provide any meaningful information related to security failures and cocaine being found at the White House," Rep. James Comer, R-Ky., chairman of the House Oversight Committee, said in a statement to The Washington Post. "The Secret Service must reassess their security operations to prevent illegal substances from entering the White House."

Arkansas Sen. Tom Cotton, the senior Republican on the Senate Judiciary subcommittee on criminal justice and counterterrorism, said he and others are suspicious in part because President Joe Biden's son Hunter Biden has acknowledged being a recovering addict.

"Someone left cocaine in one of the most secure buildings on the planet, and the Secret Service quickly determined it was impossible to find the culprit, even though a known cocaine addict lives there," Cotton said in a statement. "No serious person believes this explanation."

But Secret Service officials said the sheer volume of people visiting the West Wing during the period in question, combined with the lack of definitive evidence, made the investigation unusually daunting.

"Without physical evidence, the investigation will not be able to single out a person of interest from the hundreds of individuals who passed through the vestibule where the cocaine was discovered," the Secret Service said in a public statement. "At this time, the Secret Service's investigation is closed due to a lack of physical evidence."

The agency added, "The U.S. Secret Service takes its mission to protect U.S. leaders, facilities and events seriously, and we are constantly adapting to meet the needs of the current and future security environment."

Secret Service officials briefed members and staffers of Comer's committee on their lack of conclusive findings and their decision to close the investigation in a secure session Thursday. That decision was first reported by CNN on Thursday morning.

The cocaine was found on the ground floor of the West Wing near where visitors taking staff-led tours are instructed to leave their cellphones, The Washington Post previously reported.

The closure of the investigation comes 10 days after the cocaine was discovered, and it appears unlikely to end discussion of the matter. Cotton recently wrote Secret Service Director Kimberly Cheatle asking for more information on the agency's security procedures for the White House.

"If the White House complex is not secure, Congress needs to know the details, as well as your plan to correct any security flaws," Cotton wrote.

The small bag containing white powder was found July 2 in a cubby where guests are asked to leave phones before entering the West Wing. Described as a "dime bag" of cocaine that was only partially full of powder, the item was discovered as part of a routine security sweep conducted at the end of most evenings, according to people familiar with the matter.

The discovery prompted a brief evacuation of the building and elevated security, with a D.C. fire crew responding, as officials made sure the material was not a chemical or radiological substance. A preliminary test found the substance was cocaine, a finding that was later confirmed by a more definitive test.

Secret Service spokesman Anthony Guglielmi said at the time that the Secret Service was immediately launching "an investigation into the cause and manner" by which the substance entered the White House.

President Biden was not in the White House when the cocaine was found, but he has been briefed on the discovery. A White House official said on July 5 that the president believed it was "incredibly important" for the Secret Service to determine how it got there.

Sources said Secret Service officials reviewed logs of staffers and other who entered the White House in the weekend before the July Fourth holiday, and also reviewed videotapes of people entering, but were unable to identify any suspects and connect them to the baggie found in the cubby.