BOISE - Family and friends of Peter O'Brien are pleading for help. His brother and sister arrived in Boise Tuesday in hopes of finding their missing brother.

Expressions of concern is evident in their faces and their voices.

"I'm very, very worried because he doesn't just walk away from responsibilities," said Patricia O'Brien, Peter's sister.

Patricia and her other brother Michael O'Brien are desperate to find their younger brother Peter.

"I became extremely concerned because it's out of character for him not to turn up to collect his boys, it's just not the sort of thing he does," said Michale.

Both say they wouldn't have come from their homes in Australia and Italy if they did not think something was wrong.

"It's completely out of character," said Patricia.

Peter was last seen Thursday when he dropped his two young sons off with his ex-wife. His family says he recently moved to Boise to be closer to his boys.

When asked if Patricia thinks he may have just up and left, she responded: "I can't even imagine it, I can't imagine that scenario."

They say what else is strange in this case is Peter's door to his apartment was left unlocked, his keys, wallet with credit cards, and cell phone all inside.

"You have to be positive but the circumstances are extremely unusual, we are extremely concerned," said Michael.

Boise Police are continuing to investigate but so far Peter's family says there are no solid leads in this case of their missing brother.

Peter O'Brien is described as 5'8'', about 160 pounds, with short black hair. He has a New Zealand accent.

Anyone with information on his whereabouts is urged to call either a tip-line his friends have set up at #936-2722 or Crime Stoppers at #343-COPS.

His family says any information leading to an arrest or conviction of a suspect could be eligible for $1,000 dollar reward.