A woman with 'deep psychological problems' was in custody in the French city of Marseille on Sunday after spraying four American students with acid and burning two in the face.

The attack took place shortly after 11am on Sunday at the St Charles station, where the four women were preparing to board a train.

All were Boston College juniors in their early 20s – three of them on a study abroad program in the French capital, and the fourth based in Copenhagen, Denmark.

They were named by their university as Courtney Siverling, Charlotte Kaufman, Michelle Krug and Kelsey Kosten.

Charlotte Kaufman (left) and Kelsey Kosten (right) were two of the Boston College students attacked with acid in the French city of Marseille on Sunday

Courtney Siverling (left) and Michelle Krug (right) were also attacked in Marsielle on Sunday. The 41-year-old attacker used a cleaning substance containing hydrochloric acid that she is thought to have picked up from a local DIY store

The horrifying attack took place shortly after 11am at the Gare de Marseille-Saint-Charles station (file image)

The 41-year-old attacker used a cleaning substance containing hydrochloric acid that she is thought to have picked up from a local DIY store.

Four emergency services vehicles arrived at the station, which was packed on a Sunday morning at the tail-end of the holiday season.

After carrying out the assault, the women displayed photos of her own burns, allegedly picked up in an earlier incident.

'She displayed clear signs of suffering from deep psychological problems,' said a source close to the case, who added that no slogans were shouted out during the attack, and it was not thought to be terrorism related.

The four students were treated at a Marseille hospital but released mid-afternoon. The worst injured had suffered impaired vision.

She was one of the two hit in the face, while the other two received splashes of the substance on their legs.

The women's parents and other family members were being kept in touch with developments by the US Embassy in Paris and French officials in Marseille.

Ms Kaufman, Ms Krug and Ms Siverling are all enrolled in Boston College's Paris programme, while Ms Kosten is a student at the Copenhagen Business School.

Siverling posted on Facebook Sunday that she and her friends are all safe and said she did not sustain any injuries.

She wrote: 'Thank you so much to everyone who has reached out to see if I'm ok and/or has been praying for us. I did not receive any injuries from the attack in Marseille this morning and we are all safe. The French police and the U.S. Consulate have been wonderful and we are so thankful for that.

Four emergency services vehicles arrived at the station (file image), which was packed on a Sunday morning at the tail-end of the holiday season

Courtney Siverling posted on Facebook saying she was praying for the woman who attacked her and her friends

'I pray that the attacker would be healed from her mental illness in the name of Jesus and receive the forgiveness and salvation that can only come from Him.

'"This I declare about the Lord: He alone is my refuge, my place of safety; he is my God, and I trust him." Psalm 91:2.'

Her friend from church Savannah Freitas told DailyMail.com the three women are safe and have been helped by the U.S. consulate and the French police.

She said over Facebook messenger: 'Courtney did not receive any injuries but I did hear that the three others did but are expected to heal completely. However I am mostly struck by Courtney's incredible faith throughout this whole event.'

'As obviously traumatic as it must have been, she made it known to me personally as well as publicly on her Facebook that she is actively praying for the attacker to be healed of her mental illness.

'She so strongly believes that salvation and forgiveness comes from Jesus Christ alone and is praying that she would receive that. She is so very trusting in Lord to keep her safe and I know that everyone is so inspired by her faith in God and willingness to keep her attacker in her prayers.'

The director of the Boston College's Office of International Programs, Nick Gozik, said the women have been released from the hospital and 'it appears that the students are fine, considering the circumstances'.

French prosecutors are not investigating the acid attack as an act of terror.

Paris remains under a State of Emergency following a string of terrorist attacks carried out by Islamic State and Al-Qaeda operatives over the past two years.

Security at transport hubs including rail stations has been massively stepped up because of threats by the groups to target them.

Corrosive substances are increasingly being used as weapons in Europe, including in Britain, where there were 450 such attacks in London alone in the last year.

Acid-based products are easy to purchase, and can be paid for in cash, so there is no trace of who bought them.