Ahmed Madey Issaq, governor of the city of Baidoa, said that police raided a house on Thursday morning after a tip that a car used in the assassination attempt was seen leaving the house on the day of the attack.
Hussein Mohamed Farah Aideed, Somalia's interior minister, said five men suspected of plotting the attack are thought to have stayed in the house.
"Investigations are going on... Police are interrogating the landlord in order to get more details of the suspects," he said.
Five people were killed in the September 18 attack when a bomber blew himself up outside the parliament building in Baidoa, where the country's weak interim government is based.
Abdullahi Yusuf, the Somalian president, blamed the al-Qaeda network for the attack, in which his brother was killed.
Six fighters were also killed in a gun battle with Yusuf's bodyguards after the blast.
No one has claimed responsibility for the bombing and the country's Islamist militia, which currently controls large swathes of the country after fierce battles with the country's secular regional commanders in June, has denied involvement.
Yusuf Indahaadde, national security chairman for the group, said: "We don't have anyone in Baidoa who carries weapons or [who] wants to attack the government."
Somalia has not had an effective national government since 1991, when local commanders overthrew Mohamed Siad Barre and then fought among each other, throwing the country into near anarchy.
The country's government was formed in 2004 with United Nations aid in a bid to stem
lawlessness, however it has struggled to assert any authority.