Swedish police apprehended the man on Tuesday evening in the Stockholm area. He was taken to Nyköping and remanded in custody.
“I cannot disclose in what way he is connected to the apartment or the dead,” police spokesman Christer Sjöqvist told news wire TT.
The man was still being questioned late on on Wednesday afternoon. He is suspected of being involved in preparations to carry out 'devastation endangering the public'.
Thomas Martinson, his lawyer, told TT: “I have spoken to him and he denies all involvement. He does not understand why he is being associated with this.”
The blast, which is believed to have been caused by a bomb, took place on Monday in an apartment in the area Brandkärr in Nyköping, killing one person immediately. Another man later died in hospital.
Swedish media reported on Wednesday that the two dead men were previously known to police, but it was not known whether they had been involved in any gang violence.
Around 25 police officers have been assigned to the investigation. Police have refused to say if anyone else has so far been arrested.
"For now, we can only say that the explosion was very strong. It will take some time before we know more," Sjöqvist told newspaper Svenska Dagbladet.
News of the 30-year-old's arrest emerged as a police bomb squad was called from Stockholm just after 4pm on Wednesday afternoon to investigate a car found in the Fågelbo area of Nyköping.
"We have found a car that is of interest to us, as it could be connected to the inquiry," Sjöqvist told Swedish tabloid Aftonbladet.