It's difficult to imagine a more horrific crime than killing children as they pray in church, so it will be a particular challenge trying to understand the sort of person prepared to do it.

Robin Westman was the 23-year-old self-identified transgender woman who is accused of opening fire through the stained-glass windows of the Annunciation Catholic Church in Minneapolis on Wednesday morning during a celebratory back-to-school mass packed with children.

Two children were killed, and 17 others injured, when the shooter sprayed bullets as the young congregants scrambled down in the pews to escape the deadly fire.

Before beginning the onslaught, Westman had reportedly used wooden planks to barricade the church's two side doors shut.

Armed with a rifle, shotgun and handgun - all used in the slaughter (and, say police, all purchased legally and recently) - Westman died at the scene of a self-inflicted gunshot wound.

School shootings are in danger of losing their shock value in modern America.

And while each is tragic in its own way, there are common themes among the perpetrators that have become grimly familiar - mental health issues, easy access to guns and an obsession with emulating or surpassing previous mass killers.

The shooters often target the school where they were themselves educated, suggesting a feeling of lingering hatred and desire for revenge against their alma mater.

Westman's victims all attended the Annunciation Catholic School, where the shooter was once a pupil.

And the connection with these school shooters sometimes goes further than having been a pupil at the establishment they target.

Nancy Lanza once worked at Sandy Hook, the Connecticut elementary school where her 20-year-old son, Adam, a former pupil there, shot and killed 26 people in 2012.

So Westman's mother had previously been an administrative assistant at the school whose church service the shooter devastated on Wednesday morning.

In fact, it should perhaps come as no surprise that Westman admitted to having a particular admiration for the monstrous Lanza - who'd been diagnosed with Asperger Syndrome aged 13 and, according to his father, suffered from undiagnosed schizophrenia.

The similarities between the killers are many and chilling.

Lanza's mother, whose relationship with her son so degenerated that they were only communicating by email despite sharing the same house, was the first victim that Lanza killed.

Now it appears that this particular animus may have been shared by Westman towards mother Mary Grace – or at least the religion in which she raised her family.

Mary Grace is a devout Catholic and past anti-abortion activist who once wore a necklace of crucifixes to protest outside a Minneapolis Planned Parenthood clinic in March 2005.

Within hours of the shooting, FBI director Kash Patel announced they were investigating the killings as an 'act of domestic terrorism' and anti-Catholic hate crime.

Like Lanza, who committed one of the worst mass killings in US history, Westman had no previous involvement with the police.

And like Lanza, this was no sudden explosion of random anger and violence but a carefully planned and targeted strike against the perpetrator's former school.

Hours before the shooting, Westman shared a series of home-made videos that appeared to include a deeply twisted manifesto.

Police said the manifesto posted on a now-deleted YouTube account was timed to be released just before the attack.

It included a photo of Lanza. The Sandy Hook killer's name – along with those of other mass shooters – was written on a gun clip, or magazine, laid on what appears to be a bed, scattered with guns and ammunition.

'I have a deep fascination with one man in particular: Adam Lanza,' Westman wrote in a journal entry on May 23, which was shared in the YouTube video. 'Sandy Hook was my favorite, I think, exposure of school shootings.'

In the video, Westman showed the camera a handwritten note to family (which includes five siblings) and friends.

Messages in both English and Russian - including 'kill Donald Trump', 'Where Is Your God?' and 'for the children' - had been scrawled in white on the magazines.

In a 20-minute video, titled 'So long and thanks for all the fish' (the title of a sequel to The Hitchhiker's Guide To The Galaxy written by author Douglas Adams), a person believed to be Westman flips through a handwritten journal written in both English and Cyrillic.

One message was translated from Russian and read: 'I have had thoughts about mass murder for a long time. I am very conflicted with writing this journal.' 

The journal also included a floor plan of a church, presumably Westman's eventual target, which the reader then repeatedly stabs with a knife. 

Westman also explained motivations behind attacking the Annunciation Catholic School, where mother, Mary Grace worked until she retired in 2021.

'I am feeling good about annunciation. It seems like a good combo of easy attack form and devastating tragedy, and I want to do more research,' Westman wrote.

Another page read: 'I have concerns about finding a large enough group. I want to avoid any parents, but pre and post school drop off.'

'Maybe I could attack an event at the on-site church,' Westman went on. 'I think attacking a large group of kids coming in from recess is my best plan … Then from there I can go inside and kill, going for as long as I can.'

Westman's journal also hinted at some disturbing and confused political views which appeared to have been virulently Left-wing and anti-Semitic.

One page included a trans pride flag sticker with 'Defend equality' printed across the bottom and a sticker of an AK-47 assault rifle on top.

In another, the shooter wrote 'I hate fascism' but soon added: 'I also love when kids get shot, I love to see kids get torn apart'.

In another entry, however, Westman wrote: 'If I carry out a racially motivated attack, it would be most likely against filthy Zionist jews'.

Westman called Jewish people 'penny-sniffing' and added: 'FREE PALESTINE!'

The shooter indicated desires to carry out a 'final act' against 'a target of political or societal significance…. Targets like [Elon] Musk, Trump or some significant exec'.

The shooter applied for a birth name change from Robert to Robin in Dakota County, Minnesota, at 17 years old. 

According to court documents that name change was granted in January 2020.

The petition stated that Westman: 'Identifies as a female and wants her name to reflect that identification.'

Westman grew up in Richfield, a suburb about 15 mins drive outside Minneapolis. The shooter's mother, Mary Grace Westman, 67, originally from Lexington, Kentucky, now lives in Naples, Florida.

She joined the Annunciation Catholic School as an administrative assistant in the school's Business Office having been a secretary at another local Catholic school for seven years. At the time, she had six children (including Robin) and a grandchild.

'Mary Grace is eager to get to know everyone at Annunciation,' announced the school magazine. 'You will find that she usually has treats on her desk!'