Ho Ledinh, 65, was shot three times through the back at point-blank range outside Happy Cup cafe at Bankstown City Plaza in January 2018, in front of horrified witnesses.
He died at the scene.
Strikeforce Eugene was established by local police and the homicide squad to investigate.
They apprehended two people in the months that followed.
Analosa Ah Keni, from Liverpool, was arrested on the Hume Highway at Marulan in March and charged with murder.
It came three weeks after Arthur Kelekolio was arrested at Sydney International Airport before he could board a flight to Bali. He too was charged with murder. The pair remain before the courts.
Investigators on Thursday arrested a second man, aged 34, at a home in Leppington.
He is being interviewed by police at Narellan Police Station.
Ledinh, a father-of-five, was one of the solicitors who represented Philip Nguyen, the man jailed over the killing of Sydney police constable Bill Crews in 2010.