Dimitris Christopoulos, a university professor and prominent member of the Hellenic League of Human Rights, said at the French News Agency (AFP) the challenge prosecutors face is establishing "a link between suspected acts and actual charges".
"We are on the right track. It seems that the judicial process is accelerating, there is a certain political pressure given the present situation," he told AFP.
Christopoulos highlighted the case of Golden Dawn MP Stathis Boukouras, who was taken to a prison in his native Peloponnese -- in contrast to the other suspects, who are incarcerated in a high-security prison in Athens.
Boukouras, who has entrusted his defence to one of Greece's best-known criminal attorneys, is believed to have provided useful information to magistrates.
"Boukouras has expressed the willingness to give details on the way the party functioned," Christopoulos saidm adding that "He will operate as a Trojan horse."
Over the weekend, three more Golden Dawn lawmakers were placed in pre-trial custody, joining party leader Nikos Michaloliakos and two more senior members behind bars.
A third of Golden Dawn's 18 lawmakers are now in prison and half have been indicted on charges of belonging to, or running, a criminal organisation since the investigation began last year.
Armed with testimony from former party members, magistrates are trying to prove that Golden Dawn orchestrated attacks on migrants and political opponents, culminating in the deaths of a Pakistani immigrant and a Greek anti-fascist musician last year.
Court documents have also linked Golden Dawn to three attempted murders and numerous assaults, and witnesses have reportedly testified that senior party members were involved in extortion and possible arms smuggling.
Speaking after the murder of rapper Pavlos Fyssas in September by a Golden Dawn member, Greek Prime Minister Antonis Samaras vowed to "eradicate" the "shame" of neo-Nazism in Greece.
The crackdown was seen to have slowed by November, when two Golden Dawn members were murdered outside a party office by suspected far-left extremists.
But on the sidelines of events marking Greece's assumption of the six-month European Union presidency last week, Greek Justice Minister Haralambos Athanassiou told reporters that the investigation was "progressing steadily".
Following police raids of suspects' homes, Greek media have published a flood of photos and videos of Golden Dawn lawmakers cradling firearms, giving fascist salutes and making racist and anti-Semitic statements.
The suspects deny that Golden Dawn is a criminal organization and claim to be the victims of political persecution.
Nevertheless, the party remains Greece's third-most-popular in opinion polls, with the support of up to 10 percent of voters.