The watchdog looking into the illegal donation to Wendy Alexander's Labour leadership campaign at Holyrood has received information from her office.
Ms Alexander insists that any wrongdoing was not intentional
Labour said Ms Alexander had supplied a "huge amount" of documents, including e-mails, letters and computer files, to help the Electoral Commission inquiry.
The commission said it would review the material and would not comment further until that process was complete.
It was not clear how long that process would take.
The donation to support Ms Alexander's campaign from businessman Paul Green broke electoral law because the Jersey-based businessman is not a registered UK voter.
Mr Green said the donation was a personal cheque to Wendy Alexander, who Labour said was the regulated donee, making her liable to check the admissibility of all donations above £200.
The SNP said the matter should be referred to police.
Ms Alexander insisted she would be cleared of any "intentional wrongdoing".
Meanwhile, it has emerged Ms Alexander may not be personally accountable for the illegal donation.
BBC Scotland understands the £950 cheque was made out to the W.A. Campaign and not Wendy Alexander.
Edinburgh University public law lecturer Navraj Ghaleigh said this could make campaign treasurer David Whitton liable.
Ms Alexander's lawyers are believed to be considering this defence.
However, the Scottish Labour leader and Paisley North MSP may have already undermined this by publicly accepting that she had a legal duty to check donations and promising evidence of her efforts to fulfil this requirement.
Mr Whitton, the MSP for Strathkelvin and Bearsden, managed the finances for Ms Alexander's leadership campaign and Charlie Gordon, the MSP for Glasgow Cathcart, requested the donation from Mr Green.
Mr Ghaleigh told BBC Radio's Good Morning Scotland programme: "In legal terms, the person liable for breaching the law is the regulated donee.
Mr Whitton is awaiting the view of the Electoral Commission
"If the cheque was made out, not to Ms Alexander, but to this campaigning group, this members association, then it would appear that that organisation and its treasurer are liable and not Ms Alexander."
Ms Alexander - who has been backed by Labour MSPs - has admitted that mistakes were made, but said she would fight to clear her name.
Responding to the claims, Mr Whitton told BBC Scotland: "We've had one interpretation that says that Wendy Alexander, that the buck stops with her. Now we've got a different interpretation that says the buck stops with me.
"Frankly, I think the buck stops with the Electoral Commission. That's why we're preparing our evidence for the Electoral Commission.
"We'll give all that evidence to them, then we'll wait to hear what they've got to say."
Mr Green has accused Scottish Labour of "gross mismanagement" in its handling of the affair.