The packages contained white powder but provincial police spokesman Benoit Richard said late Wednesday that preliminary analysis has determined it was not dangerous.
The packages appeared to come from a group referring to itself as the Armed Revolutionary Forces of Quebec.
Richard said the name stems from an organization that existed nearly 40 years ago that was tied to the Front de liberation du Quebec, a nationalist paramilitary group commonly known as the FLQ that was active between 1963 and 1970.
"It's a group that has been known to the provincial police for some years but we haven't heard from them for the last, I'd say, 40 years," Richard said.
He added that it was too soon to know who exactly is behind the current incarnation of the group. Detectives will investigate the group while looking at video surveillance from the various offices to see if they can identify suspects.
The packages were sent to high-profile political and media targets.
Richard said 11 suspicious packages were delivered in Montreal alone while another 12 were mailed to other parts of the province, including to Charest's office in Sherbrooke.
"There's nothing that could lead us to believe there is a danger for the public anywhere," Richard told reporters.
The contents have been sent to a Health Canada lab in Montreal for further analysis.
At least one of the packages has been identified as actually containing baking soda. Richard said he wasn't sure that the same substance was in all the envelopes.
"I'm not ruling it out but I can't say that it is until I have the analysis done by Health Canada," Richard said.
"We've sent some pieces of it to a Health Canada lab and we'll have more information in the coming hours."
There were no reports of injuries, although some buildings were evacuated in Laval, Montreal, Quebec City and Sherbrooke.
The premier did not comment on the incident, but his office confirmed that a staffer opened the envelope which contained a letter calling the powdered substance dangerous.
Martin Carrier, a spokesman for the police in Sherbrooke, Charest's home riding, declined to comment on the contents of the letter. Montreal police and provincial police both said they'll keep the contents of the letters under wraps as part of their investigation.
The packages were addressed to six provincial cabinet ministers and another Montreal area Liberal.
Also targeted was television station TVA's Montreal headquarters; Quebecor's head office in downtown Montreal; the Loto-Quebec office tower in Montreal; the offices of Montreal La Presse newspaper and Pirate Radio, a station in Quebec City.
_ With files from Sidhartha Banerjee.