The rise in killings was not seen among people receiving care
The number of people killed by those with a mental illness increased between 1997 and 2005, official figures show.
The National Confidential Inquiry reported while 54 people were killed in England and Wales in 1997, this had risen to over 70 in both 2004 and 2005.
It was murders by people who were not under mental health care which accounted for the increase.
The head of the inquiry said the report must be kept "in perspective", but that the rise needed to be investigated.
The research, which was funded by the National Patient Safety Agency, was carried out by University of Manchester researchers.
It found that the number of suicides among in-patients had by 2006 fallen to its lowest level since data collection began in 1997 - from 219 to 141.
But there were nonetheless hundreds of suicides over that period among patients who had left a ward without permission.
The majority of these were open wards, although there were cases of patients absconding from secure units and then killing themselves.
There were no homicides by people who had absconded from secure units in the nine years to 2005.
There was however an increase in the number of perpetrators with symptoms of mental illness at the time of the killing, but not in mental health care.
These symptoms included hypomania, depression, delusions and other psychotic manifestations. Of those who were psychotic, nearly 80% had a diagnosis of schizophrenia.
The inquiry noted a rise in the number of homicides by people with schizophrenia - from 25 in 1997 to 46 in 2004.
Professor Louis Appleby, Director of the National Confidential Inquiry into Suicide and Homicide by People with Mental Illness, said: "There has been an unexplained rise in the number of homicides by people with mental illness and we now have to try to understand why this has happened.
"It is important to emphasise that the increase has not occurred in mental health patients. It is also important to keep these findings in perspective.
"The risk of being a victim of homicide in England and Wales is around 1 in 1,000 and the risk of being killed by someone with schizophrenia is around 1 in 20,000."
Marjorie Wallace, chief executive of the mental health charity Sane, said: "We are concerned by the disturbing increase in the number of people with severe mental illness, particularly schizophrenia, who commit homicide.
"While such cases are extremely rare in comparison with the general population, we believe many could be prevented if mental health services provided better care and treatment.
"These figures illustrate yet again the way in which the care in the community policy can fail to protect both patients and the public.
"In many cases known to us, the person with mental illness had shown all the warning signs before the tragedy and the families' pleas gone unheeded."
Paul Farmer, chief executive of mental health charity Mind: "Homicides by people with mental health problems are very rare, and account for only a small proportion of all homicides - contrary to the stereotypes, figures show that you have as much chance of being hit by lightning as being killed by a stranger with a mental health problem.
"Although such tragic incidents do occur, there has been no increase in homicides by people who are in touch with mental health services, or receiving treatments.
The stigma around mental health can stop people from asking for help, and this only stresses the importance of ensuring people feel they can come forward when in distress, and seek support."