07/08/2014 04:45 EDT | Updated 09/08/2014 05:59 EDT

Nathan O'Brien Missing: Calgary Police Say There Are Other Leads

CALGARY - Calgary police say a man taken in for questioning in the case of a missing five-year-old boy and his grandparents is not the only lead in their investigation.

But spokesman Kevin Brookwell isn't willing to discuss what other angles police are pursuing.

"There are other investigative leads we are following up on, so he is not the only person of interest that we are looking at," Brookwell told reporters Tuesday.

Asked to clarify, Brookwell backed away from suggesting there were other persons of interest in the case.

"He's ... a person of interest that we have interviewed, but there are other investigative leads that we are following up on."

Douglas Garland was taken into custody over the weekend for questioning in the disappearance of Nathan O'Brien and his grandparents, Alvin and Kathy Liknes, who have been missing since June 29.

Court records show Garland, 54, has a criminal past involving drugs and identity theft and Calgary police confirm his sister is in a relationship with a member of the Liknes family. Garland remains in custody on an unrelated charge.

An Amber Alert remains in effect and police are still searching the rural property and surrounding area where Garland lives north of Calgary.

Brookwell said sightings of the three missing people have been reported across the country and police still hope they will be found alive.

He said several items have been seized from the property and sent to the crime lab for testing, along with a green pickup truck matching one seen on closed-circuit video in the Liknes's neighbourhood the night they disappeared.

Nathan had been at his grandparents' on a sleepover the night of June 29 after the couple held a sale at their Calgary home on the weekend. They were selling their things as they prepared for a move to Edmonton and then on to Mexico.

When his mother went to pick Nathan up the next morning, no one was home. Police have said there was evidence of a violent incident in the house. They have not disclosed a motive.

Yellow police tape remained in place at the home in Calgary's Parkhill neighbourhood Tuesday afternoon. A lone police officer sat in his car out front. Residents of the neighbourhood just south of downtown are planning a candlelight vigil for Thursday evening.

Officers have spent a lot of time trolling a large pond and grassy fields near the Garland property north of the city.

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Nathan O'Brien, Grandparents Missing

Court records say Garland served time in prison for making amphetamines at his parents' farm.

A federal tax court ruling from 2005 shows Garland went to medical school for a year but left after having a mental breakdown.

The document says after Mounties raided his parents' property in 1992, he was released on bail and fled to Vancouver, where it says he assumed the name of a dead person named Matthew Kemper Hartley. A cemetery website lists a 14-year-old boy with the same name who was buried in southern Alberta in 1980.

Justice officials say Garland pleaded guilty to two counts of drug trafficking and was sentenced to 39 months. He received an additional one month for possession of stolen property.