Cesare Tavella, 50, was shot dead in the capital, Dhaka, in late September.
The attack was the first in a series of killings in Bangladesh claimed by the so-called Islamic State (IS).
However, the government says IS does not have a presence in the country, and has blamed local militant groups and the opposition BNP party instead.
The opposition has denied any involvement, and says the government's accusations have hampered a credible investigation into the killings.
Attacks on foreigners in Bangladesh are rare, correspondents say, but Islamist violence has surged recently.
More than 40 people, including secular bloggers, academics, gay rights activists and members of religious minorities, have been killed in attacks blamed on Islamist militants in Bangladesh since February 2013.
Five of those charged with the murder of Mr Tavella are in custody. Two, including a leading BNP politician, are still being sought by the authorities, BBC Bengali's Waliur Rahman in Dhaka reports.
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Secular bloggers, academics, gay rights activists, and members of religious minorities including Shia, Sufi and Ahmadi Muslims, Christians and Hindus have all been killed, many of them hacked to death.
A university professor whose family said he was not an atheist was murdered in April, suggesting the list of those at risk had widened further.
Who exactly is behind the attacks remains unclear. Bangladesh has myriad extremist groups and there have been few convictions over the attacks.
Many of the attacks have been claimed by so-called Islamic State (IS) or al-Qaeda linked groups.
However, the government has disputed these claims. Instead, government members have blamed opposition parties and local Islamist groups. Bangladesh's home minister has suggested an Israeli link to the killings, describing an "international conspiracy" against Bangladesh.
Both the opposition and the Israeli government have denied any involvement - and Israel described the accusations as "utter drivel".
Until the killings stop, the Bangladeshi government itself will face accusations of not doing enough to protect minorities in the Sunni-dominated nation.

Brady presented his case during a 10-hour hearing on Tuesday.
The 37-year-old was punished by the NFL (National Football League) after it was decided he was "generally aware" of a scheme to deflate footballs.
The incident happened in the Patriots' play-off win over Indianapolis Colts, which sent them to the Super Bowl.
An NFL investigation claimed the Patriots had deflated match balls to give themselves an advantage in their 45-7 victory. They went on to win the Super Bowl XLIX.
According to reports, 11 of the 12 game balls they provided for the rain-affected match were under-inflated below set standards by about two pounds per square inch, giving Brady more grip in the cold and wet conditions.
Brady was "at least generally aware of inappropriate activities", according to the independent Wells Report.
But Brady's father, Tom Brady Sr, labelled the investigation "Frame-gate", and the player's agent Don Yee said the ban was "ridiculous" and lacked "legitimate basis".
The quarterback will not receive any pay during his ban, while the team were fined $1m (£642,000).
NFL commissioner Roger Goodell presided over Brady's hearing, despite a request from the Players Association' for a neutral arbitrator.
No set time frame has been made for the outcome of the appeal.
Bangladeshi police have charged seven people, including an opposition politician, over the murder of an Italian aid worker.
[NEXT_CONCEPT]
New England Patriots quarterback Tom Brady has made his appeal against the four-match ban he received for his part in 'Deflate-gate'.