WASHINGTON - Americans have been outraged and disgusted by how some bystanders had reacted during an unusually horrific sexual assault that took place in full public view.

A man allegedly raped a woman on a Philadelphia train last week as some bystanders took videos with their phones instead of helping the woman.

Even in a nation inured to school shootings and police brutality, Americans were shocked by the brazenness of the attack, which took place on Oct 13 and was publicised at a police press conference on Monday (Oct 18).

Officials said a 35-year-old man, Fiston Ngoy, repeatedly tried to grope the woman even though she tried to push him away. He later proceeded to rip her clothes off and raped her, an assault that lasted between six and eight minutes.

Superintendent Timothy Bernhardt of the Upper Darby Township Police Department said there were several passengers in the train car, enough that "could have gotten together and done something".

Police said it appeared that some passengers "held their phones up in the direction of the assault, seemingly to film the attack".

Bystanders who failed to call the police are unlikely to be prosecuted for their inaction, a city official said on Wednesday.

Prosecutors want witnesses to come forward rather than hold back out of fear of being charged, District Attorney Jack Stollsteimer told the Associated Press.

Dr Marc Lamont Hill, a media studies and urban education professor at Temple University, expressed outrage at the reported apathy of the victim's fellow passengers.

"Now I understand that one of y'all might be scared, you don't know if he has a knife, you don't know if he has a gun," said Dr Hill on Tuesday on the Black News Channel's Black News Tonight programme, which he hosts.

"But if you've got five or 10 people on a train watching a woman be sexually assaulted and you do nothing, what does that say about you?"

He added: "Everybody's tough, everybody's talking crazy on the Internet, but when you've got a real-life opportunity to step in and stop an assault and y'all do nothing?"

But some commentators and analysts have cautioned against jumping to conclusions, noting that the police have not yet released surveillance footage of the attack.

Calling the story "mind-bogglingly horrific", CNN commentator Jill Filipovic wrote: "It suggests not only a total disregard for women's safety and well-being, but also social disintegration; a disturbing impulse to filter terrible events through the lens of entertainment and shock value, and the disturbing ability to dehumanise suffering by mediating it through a screen."

However, she also drew comparisons to the murder of 28-year-old bartender Kitty Genovese in Queens, New York, in 1964.

Police said then that witnesses had not called them or helped her, a case that formed the basis of a social psychology theory called the bystander effect, although the police account was later debunked.

Citing the inaccuracy of the police's account then, Ms Filipovic wrote: "So there are good reasons to ask questions, especially until reporters and other watchdogs are fully able to view the train's surveillance footage themselves."

Nonetheless, similar cases where bystanders have ignored women being harassed have been documented, she noted. In April, for instance, a brutal attack on a 65-year-old Filipino woman in Midtown Manhattan as security guards nearby did nothing similarly prompted outrage.

Right-wing commentators such as television host Tucker Carlson and media pundit Ann Coulter also seized on the story to argue against Democrats' immigration policies, drawing on an unverified report by the Daily Mail tabloid - which got a "mostly false" rating from fact-checking site PolitiFact - that rape suspect Ngoy was an illegal immigrant.

The American culture of recording incidents on phone cameras for social media hits, instead of intervening, has also been lambasted as a problematic practice.

Dr Hill said that while doing so was necessary in some instances such as police brutality, it was a wasted opportunity to stop an assault in others.

"This ain't for the 'gram, this ain't for Twitter, this is real life," he said, referring to social network Instagram.

"And everybody who did nothing is a coward."