The First Minister also called for a review of the Electoral Commission’s “clout” as she reacted to the news no criminal charges are to be brought in relation to the Tories’ campaign spending.
The Crown Prosecution Service (CPS) said even though spending returns submitted by some of the party’s candidates and officials may have been inaccurate, there was insufficient evidence to prove they were knowingly dishonest.
The Tories have already been fined a record £70,000 by the commission for “numerous failures” in reporting their expenses for the 2015 general election and three by-elections in 2014.
Speaking on the General Election campaign trail in East Dunbartonshire, Ms Sturgeon said: “I think the Conservatives still have some questions to answer over their election expenses from the last election.
“What we’ve heard from the CPS today is that they don’t have evidence of criminal intent but they say there may be evidence of inaccuracies in some election returns, and of course the Electoral Commission has already levied fines against the Conservatives in relation to expenses at the last election.
“I think there is certainly a doubt over the Tories about whether they were really playing by the rules at the last election and there’s still some significant questions to answer.”
Ms Sturgeon previously claimed Theresa May called June’s General Election in part because she feared numerous Tory MPs would face prosecution.
She told the Scottish Trades Union Congress last month that “we should not allow the Tory party to escape the accountability for any misdemeanours that may have led to them buying the last general election”.
SNP Westminster candidate Pete Wishart said the party backed “urgent reforms” to the elections system.
“A vote for the SNP at this election is a vote to ensure tougher rules and higher standards are introduced,” he said.
“A strong team of SNP MPs at Westminster will support new powers for the Electoral Commission, giving them the legal authority to investigate offences and providing them with much higher sanctioning powers.”
Ms Sturgeon added: “One of the lessons perhaps for all of us out of this is do the Electoral Commission need more clout?
“Because the level of fines they’re able to apply are very low and I think the time might be right to increase that substantially so that all parties know that if they breach these rules then there is a heavy price to pay.”
The Conservative Party has always maintained administrative errors were to blame for any inaccuracies, describing police complaints as “politically motivated and unfounded” .
The CPS had been considering files submitted by 15 police forces across England in relation to activities surrounding Tory campaigning in the 2015 general election.
Claims centred on whether spending on a Conservative battle bus that carried party activists to campaign in various constituencies was properly declared to the Electoral Commission as local, rather than national spending.
Parties must stay within strict spending limits in each constituency, with campaigning aimed at promoting an individual candidate counting towards local limits.
The Conservative Party chairman Patrick McLoughlin said the allegations had been “politically motivated” and were a waste of police time.
He said that “false and malicious” claims were still be circulated online and that any false statements about a candidate’s personal character during the campaign could be an election offence and defamatory.
In a statement, the CPS head of special crime Nick Vamos said: “We reviewed the files in accordance with the Code for Crown Prosecutors and have concluded the tests in the Code are not met and no criminal charges have been authorised.”
One recently-submitted file from Kent Police, relating to the election in South Thanet where the Conservative candidate, Craig Mackinlay, defeated then-Ukip leader Nigel Farage, remains under consideration, and could still result in charges.
In a statement last month, Mr Mackinlay, who has been interviewed under caution as part of the inquiry, said: “I have done nothing wrong and acted honestly and properly throughout.”
Mr Vamos said: “Under the Representation of the People Act, every candidate and agent must sign a declaration on the expenses return that to the best of their knowledge and belief it is a complete and correct return as required by law.
“It is an offence to knowingly make a false declaration. In order to bring a charge, it must be proved that a suspect knew the return was inaccurate and acted dishonestly in signing the declaration.
“Although there is evidence to suggest the returns may have been inaccurate, there is insufficient evidence to prove to the criminal standard that any candidate or agent was dishonest.”
He added: “It is clear agents were told by Conservative Party headquarters that the costs were part of the national campaign and it would not be possible to prove any agent acted knowingly or dishonestly. Therefore we have concluded it is not in the public interest to charge anyone referred to us with this offence.”
The Conservative Party was fined a record £70,000 by the Electoral Commission for “numerous failures” in reporting its expenses for the 2015 General Election, and three by-elections in 2014.
Mr McLoughlin said: “After a very thorough investigation, we are pleased that the legal authorities have confirmed what we believed was the case all along: that these Conservative candidates did nothing wrong. These were politically motivated and unfounded complaints that have wasted police time. We are glad that this matter is finally resolved.
“A number of false and malicious claims continue to be spread on the internet. People should be aware that making false claims about a candidate’s personal character and conduct is an electoral offence, as well as being defamatory.”
On being informed of the decision, Labour leader Jeremy Corbyn said he was “interested and surprised”
He said: “Our election laws must be enforced and must be adhered to. There are strict spending limits for a reason, so that money can’t buy power.”
Conservative candidates who were under investigation rounded on the Electoral Commission, with Karl McCartney, the incumbent in Lincoln, saying the organisation was “wholly unfit for purpose”.
Mr McCartney said Tory MPs would campaign to have the organisation abolished after the election and ensure that its executives “are never to play a role in our country’s public life again”.