The "reckless" poisoning of a university students' union's water supply is being investigated by police.
Floor cleaner and body lotion are believed to have been used to deliberately contaminate water at the University of Liverpool Guild of Students.
Police said there were reports of someone entering a restricted basement of the city centre building on 9 July.
The university closed the building and nobody is believed to have been harmed.
A spokesman for the university said students had been due to pick up graduation gowns from the site at the time.
Insp Geoff Stewart, of Merseyside Police, said: "Thankfully suspicions were raised at an early stage and we do not believe anyone has come to any harm as a result of this reckless act.
"But there could have had much more serious consequences."
The force has appealed for anyone who may have witnessed anything suspicious between midnight on 7 July and 15.14 BST on 9 July to contact police.
The building remains closed until 23 July.