Two men have received minor injuries in a "small explosion" at an accountancy company in Berkshire.
Police were called to Vantis, in Oaklands Business Centre, Wokingham, just after 0900 GMT when an item of mail ignited.
The building was evacuated and a cordon was set up for several hours.
Police said it was too early to say whether there was any link between this incident and recent attacks in London, Oxfordshire and Birmingham.
The police operation at the Oaklands Business Centre has been scaled down and a spokesman said the investigation would now "focus on forensic examination of the package".
South Central Ambulance Trust said two men, aged in their 30s, were treated for minor injuries but had not been taken to hospital.
Ambulance spokesman Graham Groves said: "The paramedic teams treated two male members of staff, in their 30s, for blast injuries to their hands and upper body.
"However, their injuries were not considered serious and they did not need treatment in hospital."
Fourteen people were forced to leave the building.
Ross Clarkson, a spokesman for Vantis, confirmed two members of staff had received "superficial" injuries that were treated on site.
He would not speculate as to whether the incident could be connected with Monday's "small" explosion at the Capita building in Victoria Street, central London, in which a woman received minor injuries.
Mr Clarkson said the firm, which has its headquarters in London, does tax and accountancy work for small to medium-sized businesses and is not connected with Capita.
He said to his knowledge the package had not been targeted specifically at Vantis.
Staff were sent home for the day.
Scotland Yard's counter-terrorism unit is investigating Monday's explosion in London.
A spokesman for the force said officers would examine whether there were any links between the incident and Tuesday's blast.
In the previous incidents police are investigating, one of the packages delivered to a company called Orchid Cellmark, in Abingdon, Oxfordshire, exploded, leaving a woman slightly injured.
All the companies received jiffy bags containing a crude firework-type explosive.
Each had a similar return address and a well-known animal rights activist, Barry Horne was named on the back of one envelope. Barry Horne died in 2001.