There were dramatic developments in the head-in-the-bag murder mystery last night as detectives were believed to be questioning a man over the death of drug dealer Stephen Ramon Cookson.
The arrest came hours after forensic officers focused on a beach shack north of Mandurah, where two suspects may have handled Cookson's body believing it to be the remnants of a clandestine drug laboratory.
Police said homicide squad detectives were questioning a man over one of their inquiries but would not say which one.
The man was photographed leaving homicide squad headquarters in Northbridge about 8pm, and it is not known whether he had been charged or what his role in the gruesome case may be.
It is understood Cookson was murdered at his East Perth apartment, in Hay Street, about a fortnight before Christmas.
_The West Australian _has also learnt that one of the last sightings of the drug dealer was in a TAB near the apartment complex about three weeks before his head washed up on a Rottnest Island beach.
Yesterday, _The West Australian _revealed at least two men believed to have been involved in the disposal of Cookson's body, but not his murder, had spoken to police.
Detectives have quizzed several suspects, including the two men, who were interviewed for up to 10 hours on Tuesday.
They may have had no idea the bags they were handling contained Cookson's remains.
Cookson's head was wrapped inside two plastic bags and was found by an 11-year-old girl beachcombing with her family on January 6.
Police would only say the Golden Bay shack, in Ellendale Street, was a site of interest to detectives investigating Cookson's murder.
But the scale of the forensic search suggested police believe it is a key location in the hunt for his killer.
It is believed Cookson was dismembered and some or all of his remains were thrown in the ocean.
Only his head has been found.
A source told _The West Australian _ that police had information this week that Cookson's remains had been dropped in the ocean somewhere between Mandurah and Rockingham and that there was air in the bag containing his head.
Police were also called to a Silver Sands beach after a girl walking with her mother spotted what she believed were human remains.
Forensic experts took the material in for testing but it is believed it came from a marine animal.
Neighbours on Ellendale Street said they believed the shack was a holiday home that was infrequently used by the same group of men, most recently for a party shortly before Christmas.
One resident said detectives had asked whether they noticed anything suspicious at the property between December 17 and 20.
Resident Errol McMahon said the occupants came and went, and the last time he saw them was before Christmas when they had a party.
Asked what he thought when he saw the large police presence, Mr McMahon said: "I thought it was a drug lab at first."
Mr McMahon did not recall seeing Cookson at the property. He said he did not have a problem with the neighbours but never had a reason to speak to them.
"Every time I saw them they were just some boys drinking beer," Mr McMahon said. "I never saw anything untoward."
Police parked a semitrailer that will be used as a command point on a vacant block opposite the Ellendale Street home. Teams of forensic experts started the painstaking task of mapping the unkempt backyard. They were also searching the backyard of an adjoining property because there was no dividing fence.
Major crime squad detectives were canvassing neighbours about movements at the yellow fibro home, which had a lounge suite on the front veranda overlooking the street.