VANCOUVER - The two police agencies that failed to stop Robert Pickton as he hunted sex workers in Vancouver's Downtown Eastside are about to appear at the public inquiry into the case to explain why they were unable to catch a serial killer.

The inquiry has spent weeks hearing expert evidence about the Downtown Eastside and the city's sex trade, as well as testimony from the victims' families. But its main purpose is to examine what went wrong with the police investigations.

That work is set to begin Monday, as Vancouver's Deputy Chief Doug LePard, who authored an internal report about the Pickton investigation, becomes the first police witness. After him, the author of a similar RCMP report will testify, as will a senior officer from the Peel Regional Police in Ontario who was asked to provide an outside opinion.

The testimony will mark a significant turning point at the inquiry, which will now spend the next several months hearing from police officers.

"We've set the stage and provided the backgrounds, and now we'll do the main work, which is the police investigation," Art Vertlieb, the commission's lead lawyer, said in an interview.

Once the authors of those three reports testify, the inquiry will then hear from officers who were actually involved with the investigations, said Vertlieb.

Commissioner Wally Oppal has been asked to examine why the police failed to catch Pickton as he murdered sex workers in the late 1990s and early 2000s, and why prosecutors declined to pursue attempted murder charges against Pickton after an attack on a prostitute in 1997.

Oppal's job will be to sort through the testimony of investigators and the inevitable finger-pointing between the Vancouver police and the RCMP to determine precisely what happened, and then make recommendations for the future.

Pickton was arrested in 2002 and eventually convicted of six murders, though the remains or DNA of 33 women were found on his farm. Pickton claimed he killed 49.

Vancouver police were investigating reports of missing women in the Downtown Eastside, while the RCMP in nearby Port Coquitlam, where the Pickton farm was located, started looking at Pickton after the attempted murder allegations in 1997. The two forces later formed a joint investigation.

The Vancouver police has already admitted its investigation was a failure and apologized to the families of Pickton's victims. The RCMP has done neither.

Both forces have argued that, regardless of any failings the inquiry identifies, their officers did the best they could with the information they had at the time.

LePard will take the inquiry through his 450-page report, which was released publicly last year and detailed a series of errors made by the Vancouver police and the RCMP.

LePard blamed ineffective information sharing, poor leadership and a lack of resources. He said several officers who came forward with information were ignored, particularly Kim Rossmo, a geographic profiler with the Vancouver police who warned a serial killer could be at work.

The report saved its worst criticism for the RCMP, which LePard accused of letting the investigation sit idle for months and then botching an interrogation with Pickton in 2000.

The RCMP's internal report, which was kept hidden until it was submitted as an exhibit at the inquiry last month, is much less critical.

Prepared in 2002 in response to a civil lawsuit, the report admits there were difficulties in corroborating allegations that Pickton was involved in killing sex workers, but nevertheless suggested the force did all it could.

The report also insists the RCMP was able to work well with the Vancouver police, and it complained that scarce resources were spread across a number of high-profile investigations, making it difficult to pursue Pickton with more vigour.

The Peel Regional Police report hasn't yet been submitted to the inquiry.

Jason Gratl, an independent lawyer appointed to represent the interests of the Downtown Eastside, said the reports from the Vancouver police and the RCMP are useful, but he said the inquiry must be careful to treat them with an appropriate level of skepticism.

"We don't want to see the LePard report serve as a substitute for the commissioner's role in making findings of fact, but the LePard report and his testimony is likely to be useful to provide a broad overview of what happened in terms of the investigations," said Gratl.

"From our point of view, it's difficult to judge prior to cross-examination the extent that these reports were intended to serve institutional interest."

Cameron Ward, who is representing the families of 18 missing women, said he's more interested in hearing from the police officers themselves, rather than the authors of internal reports.

"What my clients are really interested in probing is what the investigators themselves did or didn't do, what they knew and when they knew it, and why it took them almost five years to stop Mr. Pickton," said Ward

"Those are the key factual questions, and I don't think we're going to get answers to those until the actual investigators show up,."

The inquiry will take a break at the end of the month and resume in the new year.

Oppal hopes to finish hearings by the end of April in order to finish his final report by his deadline of June 30.