RAB said he was taken to the National Institute of Cardiovascular Diseases (NICVD) on Thursday afternoon after he had complained of chest pain.
He died in custody at a time when students are demonstrating for a fresh medical and dental admission test, alleging the one held on Sep 18 was taken on leaked question papers.
Apart from Siraj, the RAB during its raid that day had arrested Bangladesh Judicial Service Commission (BJSC) storekeeper Rezaul Karim and a youth identified as Ishan Imtiaz Hridoy.
After their arrests, RAB claimed to have seized 23 answer sheets and two question papers of the BJSC 2014 assistant judge recruitment test.
It also recovered Tk 200,000, cheques of Tk 400,000, seals of several government organisations, cheque books of several banks, three mobile phones, and an iPad.
The elite police unit also said Omar Siraj was the kingpin of a ring that provided students with answers with the help of Bluetooth earphones.
It said the group helped the candidates during test for admission to MBBS, BDS courses and for recruitment to Bangladesh Krishi Bank, BJSC and other government organisations.
The UGC later had temporarily suspended Omar Siraj.