The former head of Halton police's drug and firearms unit has been arrested and now faces several charges following a seven-month investigation — and an audit finding dozens of exhibits involving opioids had allegedly been "tampered with."

Staff Sgt. Brad Murray, a 16-year member of the Halton Regional Police Service, was a member and supervisor of the Drug and Morality Unit from January 2013 to May 2016.

Murray was arrested on Sunday, and is now facing various charges including two counts of breach of trust, two counts of theft under $5,000, and obstruction of justice.

As a result of the investigation, a "number" of drug-related trials have been stayed by the prosecution, according to a Halton police news release.

"The full ramifications of this situation as it relates to court prosecutions and potential withdrawal of charges will not be known for some time," the release reads.

'Tampered with' exhibits all involved opioids, police allege

Halton police say that in the fall of 2016, a "drug exhibit anomaly" was discovered by members of Murray's former unit, which is responsible for enforcement of all drug, vice, and firearm matters in Halton region.

As a result, the force started a comprehensive internal audit of all drug exhibits seized by Halton police.

The audit revealed that roughly 30 exhibits had allegedly been "tampered with," all of which involved prescription or illicit opioids such as OxyContin, say police.

"The exact number of exhibits affected will be a matter for disclosure during the court process," according to the police news release.

Halton police chief Stephen Tanner contacted Toronto police in November 2016 and asked for an independent, external criminal investigation relating to the exhibits. This is "common practice" in matters pertaining to internal criminal investigations and ensures an impartial and transparent investigation, notes the release.

Toronto police began their criminal investigation in December 2016, which led to Murray's arrest. The alleged offences happened during Murray's time with the Drug and Morality Unit.

Suspended with pay

Murray has been suspended from duty with pay, as per the Police Services Act, according to Tanner. Halton police will also be conducting an internal disciplinary investigation.

Murray will be facing disciplinary procedures in accordance with the act in addition to criminal charges, say Halton police. The most serious penalty under the act is dismissal.

In a statement, Tanner said the situation "violates public trust in the work we do and is an affront to the Canadian justice system as a whole."

He said the Halton police will continue to support the decisions of the Toronto police, Public Prosecution Services of Canada and the Ministry of the Attorney General's Office.

"In light of the findings and going forward, the Drug and Morality Unit has implemented additional measures to preserve the integrity of all seized drug exhibits," Tanner said. "I am confident in the security of our service's drug exhibits and that no other cases or types of drugs are in question."

A spokesperson for Halton police said no further comment will be made as the matter is now before the courts. Toronto police told CBC Toronto they will also not be commenting.

Murray will be making a first court appearance in June.