Counter-terrorism police investigating so-called "Punish a Muslim Day" letters have been given more time to question a 35-year-old arrested on suspicion of conspiracy to murder.
The detained man from south Lincoln was this morning 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.
Read more: Man arrested on suspicion of conspiring to murder after 'Punish a Muslim Day' letters
Detectives have until 6.20am on Friday to question the arrested man.
The letters were sent out in March to cities across the UK, including locations in West Yorkshire.