A senior source said: "Gardai have nothing to suggest that this attack was carried out by a crime gang. Instead, it seems to be the work of a deranged person.
"The person got entry into the building by ringing a buzzer. The secretary who let the person in is not 100pc sure but thinks it may have been a woman.
"When the person failed to appear in the office, the secretary went out to have a look and she saw smoke. It was a small enough fire caused by petrol and some rags -- it was put out very quickly.
"CCTV is being studied today in an attempt to identify the culprit. All aspects of this case are being investigated including whether this was the work of an irate constituent."