A 33 year old man was arrested in Omagh.
A 57 year old man was arrested in Belfast earlier today.
Both were arrested under Section 41 of the Terrorism Act.
They have been taken to Musgrave Serious Crime Suite for questioning.
A 71-year-old arrested over the inquiry has now been released following questioning.
A 47-year-old arrested in connection with the attempted murder remain in police custody at this time.
Four other men arrested over the murder bid have since been released unconditionally.
DCI Caldwell remains in a critical but sedated condition in Derry’s Altnagelvin Hospital.
The New IRA has claimed responsibility for the murder bid.