Counter-terrorism police investigating so-called 'Punish A Muslim Day' letters have been given more time to question a man arrested on suspicion of conspiracy to murder.
The 35-year-old from south Lincoln is in custody in West Yorkshire following his arrest on Tuesday.
Lincolnshire Police said the man had been arrested on suspicion of sending a hoax noxious substance, sending threatening letters and conspiracy to commit murder.
Detectives have until 6.20am on Friday to question the arrested man.
Searches have taken place at a residential property in south Lincoln and an office building in the city centre.
It follows a number of reports of hate mail being received in March.
A police spokesman said: "We understand that people may be concerned following this activity.
"However, we would like to reassure communities that this activity is a result of an ongoing, intelligence-led investigation and was not in response to an immediate threat."