British authorities have struggled in recent years with a troubling rise in acid attacks that have left many victims blinded and disfigured.
Some have been part of gang violence. Some have been linked to broken relationships. And some defy easy explanation for their apparent randomness.
Officials now are trying to understand why a 3-year-old boy was splashed with a caustic substance Saturday in a discount store in the English town of Worcester. The toddler suffered severe burns on his arm and face. He has since been able to leave the hospital, but authorities now say they believe it was a deliberate attack. Four men have been taken into custody.
"The incident will rightly shock the local community, and I would like to reassure local people that we are carrying out a thorough investigation," said West Mercia police Chief Superintendent Mark Travis.
Following previous attacks, investigators have at times been at a loss to find a pattern.
Many attacks have occurred in or around London and have targeted gang members. But a range of others - from bankers to moped riders - have become victims in apparent random attacks.
Stopping acid attacks also is difficult, mostly because acids can be easily purchased in convenience stores as drain cleaners or bleach.
Such attacks mostly target women and are carried out by men in parts of South Asia where they are particularly prevalent. Data by London Metropolitan Police last year found that attackers and victims in Britain are predominantly male. The overall majority of victims is younger than 30 years. Among the attackers, every fifth identified suspect was a minor.
In London, the number of attacks was at an all-time high last year, with 50 percent more cases reported than in 2016. (Three of the four suspects in custody over Saturday's attack were arrested in the British capital, even though the crime occurred more than 100 miles away.)
In some districts, courts have started asking visitors to take sips out of their bottles if they wish to enter with liquids to prevent attacks.
On a national level, authorities are also considering the introduction of new guidelines and laws to restrict sales of liquids that could be used in attacks or to introduce harsher minimum punishments.