Story highlights Shafilea Ahmed's parents are accused of killing the 17-year-old in an "honor murder"

They denied killing her, but on Monday, the mother said she saw her husband attack the girl

Shafilea's sister testified last month that she saw her parents stuff a plastic bag into her mouth

The United Nations estimates there could be 5,000 honor murders per year worldwide

Shafilea Ahmed's parents have long denied killing their pretty teenage daughter for shaming the family, but the "honor murder" case that has gripped Britain took an unexpected turn this week.

Shafilea's mother, Farzana, abruptly changed her story in court and implicated her husband, Iftikhar, in the killing, CNN affiliate ITV reported Monday.

Iftikhar and Farzana Ahmed are on trial in Chester, England, accused of killing Shafilea, 17, (pictured left) in September 2003. They have pleaded not guilty.

Her dismembered body was found on a riverbank months after she disappeared. She had been stripped of anything that would identify her, prosecutor Andrew Edis told the jury in May, according to ITV.

Newspapers, television and radio have all been reporting on the prosecution case that Shafilea's parents killed her because they felt her "Western" lifestyle brought shame on the family.

Shafilea's sister Alesha testified last month that she saw her parents kill the teen by stuffing a plastic bag into her mouth.

She said her parents were angry that Shafilea was wearing a short-sleeved, V-neck top, and no sweater, on the night she was killed.

"Just end it here," Farzana said to Iftikhar, according to their daughter.

They pushed Shafilea down onto a sofa and suffocated her despite her struggles, Alesha testified.

JUST WATCHED Parents accused of 'honor killing' child Replay More Videos ... MUST WATCH Parents accused of 'honor killing' child 02:50

Prosecutor Edis called it "an act of suffocation by both parents acting together."

Alesha Ahmed did not tell police she had seen the killing until 2010, after she was detained by police in connection with a robbery of the family's home, ITV reported.

She has pleaded guilty to robbery. Prosecutors said she had not been offered any sort of deal in exchange for testifying against her parents.

She has testified that both her parents physically abused Shafilea "every day" over the course of five years, and that her mother did it more "because she was at home more."

On Monday, Farzana unexpectedly said she had seen her husband attack Shafilea. She said that she tried to intervene to protect the girl, but that her husband pushed her away and punched her, ITV reported.

"Extremely scared," she fled the room and stayed in a bedroom with other children until she heard a car leaving 20 minutes later.

When her husband returned alone, she asked where their daughter was.

"If you care for your dear life and that of your children, don't ever ask me this question again," he told her, ITV reported.

Farzana Ahmed testified Monday that only one of their children, Mevish, was present when she saw her husband attacking Shafilea.

The jury was told that Shafilea had been taken to Pakistan for a hastily arranged marriage before her death, and drank bleach there when her parents suggested she was staying there when the rest of the family returned to England, ITV reported.

She was hospitalized for three months after the family came home because of the bleach incident, Alesha testified.

Alesha testified in May that her parents were very strict with their daughters, ITV reported.

"Our family was very restricted. It was very different. The Pakistani culture is more restrictive in terms of what you can wear or do," she said.

The trial began in May and is expected to last several more weeks, prosecutors say.

Reliable figures of the number of honor murders around the world are hard to come by, but the United Nations Population Fund has estimated there could be 5,000 per year.

So-called honor murders are a significant enough problem in Britain that the country's Crown Prosecution Service has an expert specializing in cases where members of a family kill relatives because of behavior that they say shames the family.

The CPS began keeping statistics on honor violence in April 2010 and prosecuted 234 cases in the following year, just over half of which were successful.

The CPS expert, Nazir Afzal, told CNN earlier this year that convicting perpetrators can be difficult.

"There is a wall of silence around this, and people are not prepared to talk," he said.

But Afzal insisted that it was "absolutely important that you bring every single person to justice, because you want to deter other people from doing it."

There is a perception that the crime is particularly common among Muslims, but one vocal British campaigner says not all honor violence is perpetrated by Muslims.

Jasvinder Sanghera, who was the victim of a forced marriage, is not Muslim; she is Sikh.

"Significant cases are happening within South Asian communities, be it Pakistani, Indian, Sikh, Muslim, Kurdish, Iranian, Middle Eastern communities," she said.

Afzal says "no faith on Earth" justifies killing.

"At the end of the day, murder is murder," he said. "Abrahamic faiths say 'Thou shalt not kill.' At the end of the day, nobody should die for this."

The killings take place in many parts of the world, experts say.

"It's definitely a problem that happens in many different places: the Middle East, Pakistan, Bangladesh and among immigrant communities in North America," said Nadya Khalife, a researcher on women's rights in the Arab world for Human Rights Watch.

Several Arab countries and territories, including Iraq, Kuwait, Syria, Yemen and the Palestinian territories, have laws providing lesser sentences for honor murders than for other murders, Human Rights Watch says.

Egypt and Jordan also have laws that have been interpreted to allow reduced sentences for honor crimes, the group says.