U.K. police launched a hate crime investigation after anonymous letters threatening acid attacks were sent to Muslim homes in Bradford.
The threatening messages contained images of a sword and St. George's flag, and read: "Kill scum Muslim."
The letter questioned why Muslim women wore burkas, then vowed to "do acid attacks on anyone who wears the funny black masks around your square & Bradford & other places."
The letter also said that three supposedly Muslim men, who it called "three of ur [sic] male pigs," had made advances on four white girls, continuing, "We know who the three male pigs are they are walking dead pigs."
West Yorkshire police said they were taking the threats "extremely seriously" and had increased patrols in Hanover Square, which is a predominantly Muslim area in the inner-city.
The police added that they were working with the local community to identify and prosecute the perpetrators.
"We understand the impact hate crimes and hate incidents can have on our communities and on individuals, and crimes of this nature will not be tolerated," the police said.
Meanwhile, local Muslims said they were shocked and frightened by the letters.
"I was shocked. My mum wears a burqa and she goes to town regularly so I was concerned for her. When I explained it to her she realized the severity of it and was afraid. You start wondering whether it's safe to go out on their own," one of the recipients was quoted as saying by The Guardian.
Another recipient assumed that Muslims could be targeted during the English Defence League march Saturday, which coincides with the holy Muslim feast of Qurban Bayram (Feast of Sacrifice), also known as Eid al-Fitr.
"Eid could be Friday or Saturday – a lot of times we'll all be out to families, people go out to restaurants in the town center. There's always that chance something could happen," he said.
Anti-Muslim attacks in the U.K. have skyrocketed following deadly terror attacks in Manchester and London, according to official figures.
Acid attacks have also spiked in the country in recent years. London police say the number of reported attacks with corrosive liquids rose from 261 in 2015 to 454 in 2016.
In July, a young Muslim woman and her cousin suffered "life-changing" injuries after an acid attack in east London when she was celebrating her 21st birthday. Resham Khan and her 37-year-old cousin Jameel Mukhtar were doused in sulphuric acid by an attacker, who was identified as a man named John Tomlin, while they were inside a car in traffic in Beckton. Tomlin was later arrested and charged with two counts of grievous bodily harm with intent.