As per reports, a senior police officer said two persons were identified for allegedly lobbing the grenade from a bike.
Based on CCTV footage, the police recovered the bike and then arrested one accused Hiren Gogoi alias Robin from Lakhipathar under Digboi police station on Monday.
A bounty of rupees 2 lakh was announced to catch the culprits.
Gogoi is a surrendered ULFA militant and his name was mentioned in some extortion cases, a PTI report stated.
After interrogating Gogoi, the police arrested the second accused Ranjit Changmai of Lakhipathar.
The duo do not have any direct link with ULFA(I), but the police are investigating the motive behind the attack, a police official was quoted as saying in the PTI report.
On May 14, two people were killed while another person was injured in a grenade blast by suspected ULFA(I) in the market area at Tingrai under Digboi police station in Tinsukia district.
Meanwhile, Assam Chief Minister Dr. Himanta Biswa Sarma announced rupees five lakh ex-gratia to each family of the two killed in the incident.