Ryan Keating knew why I had called.

“Yes, I’m the interim CFO in question.”

Keating is founder and managing partner of Keating Consulting Group, a 14-year-old Silicon Valley firm that has provided outsourced CFO and accounting services to such companies as Marketo, Nest and Snapchat. Also on that list is Zirtual, the on-demand virtual assistant company that collapsed on Sunday night, laying off more than 400 employees without a word of warning.

Earlier today, Zirtual founder and CEO Maren Kate Donovan told Fortune that her company had been severely damaged by a outsourced CFO firm that had worked with Zirtual until this past April. She wouldn’t name the firm, but said that it had provided “burn rate” projections that did not take into account how the company would have three pay periods (rather than two) in May and October due to calendar anomalies. Or, in Donovan’s words: “The numbers were just completely fucked.”

A source provided us with Keating’s identity, and he readily acknowledged his involvement. In fact, Zirtual even remains listed as a client on his firm’s website.

 

Keating says he “feels horrible about what happened to Zirtual and its employees,” and that he’s been stunned by the past week’s events. He also expressed fondness for Donovan, but added that he had not spoken with her in weeks.

As to those burn rate projections, Keating expressed surprise that Donovan is using them to explain the company’s collapse. Yes, he created Excel spreadsheets that only had two pay periods per month (i.e., 24 pay periods rather than 26). But he insists that the cash projection in each pay period was artificially higher (on a sliding scale) to make up the difference, thus meaning that Zirtual shouldn’t have suddenly had to come up with hundreds of thousands of unexpected dollars.

“These are projections and they’re never right on the nose but I can’t imagine that this had a big impact in them getting through August 15,” Keating says.

To Keating, the real culprit here might have been a business model that no longer made sense. First, Donovan chose to transition the company’s employees over from independent contractors (i.e., the Uber model) to fulltime employees (complete with benefits). At the same time, however, she continued to insist that each of the virtual assistants — or ZAs, as Zirtual called them — be U.S.-based and college-educated, so as to provide a superior service to clients.

“In hindsight it was a much bigger change financially than was originally anticipated,” Keating explains. “Maren had all the right intentions, but it really cut into margins and the benefits meant that we could only pay the ZAs $11 or $12 per hour. That created a huge amount of turnover among a U.S.-based, college-educated employee base, with lots of people dropping out even during the training program. And what that meant was that Zirtual had to project hiring 20-30% more people than it actually needed, so that it wouldn’t be caught short-staffed.

Keating adds that the company always knew that it would need to raise money in the summer of 2015. But he had not readily kept up with the funding’s progress since his firm stopped working with the company.

What seems to have happened next is that Donovan was too trusting of what she thought was a bird in the hand.

Multiple sources say that Zirtual tried raising a $3 million funding round at a $38 million pre-money valuation, with Donovan telling Fortune that the new capital would be used to cover payroll and to increase ZA pay to $16 per hour (thus reducing employee churn). What proved more difficult was finding the remainder. One venture capitalist tells me that he was contacted a few weeks back, but that the dwindling cash reserves raised too many red flags. Other venture capitalists who invest in on-demand startups say they were never called.

Zirtual did seem to find a backer — described only as a “micro VC firm” — to fill out the round, but it pulled out last Thursday (one theory, and it’s only a theory, is that the new investor hadn’t originally realized the extent of the cash crunch). There has been some criticism of Donovan for being interviewed last Friday in an online program hosted by Jason Calacanis — an angel investor who had backed Zirtual — but Calacanis says the interview was actually taped two days earlier.

The company and some of its existing investors (including Mayfield) made frantic calls into Friday night, but to no avail. On Saturday, Donovan’s only other board member quit. By Sunday, it was over.

Startups.co on Monday agreed to acquire Zirtual, although sources told me that, as of last night, the deal hadn’t officially closed. It’s an all-stock fire sale with some potential upside for Zirtual investors if the thing turns around. No decisions yet on how many ZAs will be rehired (as contractors this time), or how much of the 40-person executive/operations team will be retained (including Donovan).

Startups.co CEO Wil Schroter says he learned about the opportunity when, as a Zirtual client, he got the service pause notice. He also acknowledges having done relatively little due diligence into the company’s books, believing that speed was of the essence in order to retain clients who are being wooed both by other startups and by some ZAs who are going into business for themselves. For example, Schroter does not know the outside investor that bailed, or why.

As for Keating, he continues to support Donovan. “She was a great CEO. It just became very hard to get on top of the business model.”

Leigh Gallagher (@leighgallagher) contributed to the reporting of this story.

An earlier version of this story incorrectly stated that Zirtual was hoping to raise per hour salaries to $18 per hour. The correct figure is $16.