Stepping Hill deaths: A look inside 'secure' hospital
29 March 2014 Last updated at 19:58 GMT
The BBC has been granted access to Stepping Hill hospital in Stockport, which has increased its security.
On Saturday, a nurse appeared in court charged with murdering three patients there in 2011.
Victorino Chua, 48, of Churchill Street, Heaton Norris, is also charged with 31 other offences including GBH and attempted poisoning.
Ed Thomas reports.