The Dhaka Metropolitan Police (DMP) has been trying to determine the source of explosives that were used to set fire to buses in the capital, said sources.
Police have already arrested 32 leaders and activists of BNP in 14 cases lodged over the arson attack.
Walid Hossain, deputy commissioner (media) of DMP, said, "Investigators are trying to trace the source and the routes by which the explosive reached the spots."
He said police are analysing CCTV footage and getting experts' opinions to find out who the arsonists are.
Abu Bakar Siddique, officer-in-charge of Paltan police station, told The Business Standard that, "One bus in the Paltan area was set on fire with petrol bombs, but we do not know which explosives were used to torch the other bus in the area."
Meanwhile, Home Minister Asaduzzaman Khan Kamal on Saturday issued a note of warning that stern action will be taken against those involved in arson attacks on buses in the city.
"None will be spared…those who are involved in violent activities as well as those setting fire to buses," he told journalists at his official residence in Dhaka.
The minister said BNP could not accept the by-polls result because its candidate was defeated in the Dhaka 18 by-polls.
Law enforcement agencies have said they received some conversations among BNP leaders relating to the arson attacks on buses, which prove BNP's involvement in violent activities.
Kamal said a peaceful situation has been prevailing in the country because law enforcement agencies are now actively maintaining law and order.
According to the DMP, 28 of the accused arrested in connection with the arson attacks were remanded.
Police have increased their patrols and number of check posts in the capital to avert untoward incidents.