New Zealander Cathy Marlow’s sister Debbie rose to her feet in the Old Bailey court house in London on Tuesday and screamed “Yes!” as American Matthew Fagan was sentenced to serve a minimum of 26 years in jail for killing Marlow.

Marlow’s father, Bernie, and other members of her family flew over from New Zealand for the trial, the BBC reported. They were clearly pleased when the verdict was reached.

A 28-year-old finance manager, Marlow was beaten over the head with a blunt instrument by Fagan when she caught him stealing computers. He then throttled her with a red scarf knitted by her mother.

She had gone into work on a Saturday to catch up after a holiday to Egypt. Her body was found in a shower cubicle at the offices of Research Now in Stockwell, south London, in January last year.

Fagan, 33, of Bermondsey, south east London, had admitted burglary but denied murder.

Fagan said he was desperate for money and facing bankruptcy, and struggled with Marlow when she caught him stealing. He told the Old Bailey he helped tie her up but she was alive when he left her with an accomplice.

Richard Whittam QC, prosecuting, said: “It was her diligence going to work on that Saturday that cost Cathy Marlow her life.”

The court was told the 28-year-old had arrived at the office at 9.15am and had arranged to meet a friend for lunch. At 1.30pm she spoke to her friend and said she would be leaving soon, but she never arrived.

The friend contacted Simon Edwards, who worked at Research Now. They found her body and immediately called the police.

The walls and floor of the office were smeared with blood. Detectives sent away material for forensic analysis and a DNA sample from underneath Miss Marlow’s fingernails gave them the match — Fagan.

He had had a DNA sample taken after he was arrested for being drunk and disorderly, and detectives discovered he had worked at Research Now as a web production manager before being fired in April 2006.

Fagan and Marlow knew each other by sight and detectives realised he must have panicked and killed her when she disturbed him in the office.

After killing her, rather than aborting the burglary, he continued.

His rucksack was seen on CCTV footage, bulging with stolen laptops as he left the office.

Fagan was born in Oregon and grew up in a small logging town, White Salmon, just across the border in Washington state.

At the age of two he moved to Canada — but he returned to White Salmon, aged 14, and graduated from Columbia High School in the town.

The school’s assistant principal, Janet McCutcheon, said: “He did not stand out and was not in a lot of trouble. He was just an average guy.”

She said he had been enrolled in her Future Business Leaders of America class.

When he gave evidence, Fagan, speaking in a strong American accent, said he had been married since 2003 to a German woman.

He admitted falling heavily into debt after losing his job and filing for bankruptcy.

Fagan said he had been earning cash working for a removal company and carrying out burglaries on the side.

He thought his former employers would be ripe pickings for a burglary.

Fagan told the court that when Miss Marlow had found him and his accomplices in the office they had grabbed her, muffled her screams and tied her up.

He claimed she was alive when he left her to meet other accomplices.

On the way home Fagan said he had been mugged and injured by a man carrying a knife. He said he had then gone to Guy’s Hospital to get his injuries treated.

But Detective Chief Inspector Damian Allain said Fagan was a “devious and callous” man who had told a tissue of lies.

Richard Whittam QC, prosecuting, told a jury: “It was her diligence going to work on that Saturday that cost Cathy Marlow her life.”