Police are investigating two suspicious packages which are thought to have been posted in Greater Manchester.
One of the packages was intercepted at a sorting office
Each contained a letter and a miniature spirit bottle containing liquid with a high concentration of caustic soda.
The packages were sent to a councillor in the Blackburn area of Lancashire and a journalist in Glasgow.
One of the letters was signed "SNLA". It is not known if it was from the Scottish National Liberation Army - a group pursuing Scottish independence.
Greater Manchester Police have stressed the two suspect packages did not contain explosives and that there was nothing to suggest any link to international terrorism.
However, caustic soda in high concentrations can cause burns to the skin and breathing problems.
Assistant Chief Constable Dave Thompson said it was not known whether any more packages were in the postal system.
"Because of the way it was contained, there's a risk that the substance could be consumed," he said.
The force said the package addressed to the councillor was intercepted at a sorting office, but the package addressed to the journalist was delivered.
Mr Thompson would not say what was written in the letters, or why the two individuals had been targeted.
"Because of the nature of the investigation we don't want to go into any details about precisely what was contained in the letters," he added.
Mr Thompson confirmed that the initials SNLA was written on one of the letters.
"At this stage the only indication is an initial used on the letter so we can't at this stage - in any shape or form - identify who is responsible for that, or whether indeed there is any allegiance to such a group.
"At this stage we are carrying out an investigation into the circumstances around these postal items. We cannot say why these particular individuals have received them."
In 2002 a man claiming to be a member of the Scottish National Liberation Army (SNLA) has said he sent two toxic packages to Downing Street and a Scottish MP.
Dubbed "tartan terrorists", the SNLA is one of a number of fringe organisations seeking independence for Scotland.
It was formed by a former soldier with the Argyll and Sutherland Highlanders called Adam Busby and engaged in low-level violence against English "settlers" in the 1970s and early 1980s.
In 1983 there were 27 SNLA attacks, including letter-bombs to Margaret Thatcher and the Princess of Wales.
Eight years later the organisation sent Prince William a fake anthrax bomb at St Andrews University.
The group is now thought to be made up of a small number of militant extremists and individuals claiming to be members have been jailed for letter-bomb offences.