Fallen financier and former New Jersey governor Jon Corzine said he was "stunned" by the collapse of his firm MF Global but told a congressional panel that he had no idea where the missing $1.2bn (£780m) had gone.

Corzine faced hostile questioning after being subpoenaed to testify before the House of Representatives' agriculture committee on the bankruptcy of the financial firm and the missing money – some of which is owed to farmers who used the company to hedge crop investments.

The former Goldman Sachs chief and one-time confidant of Barack Obama was the first former senator to be subpoenaed to appear before Congress since 1908.

An exhausted-looking Corzine said he was "devastated" by MF Global's collapse but dodged specific questions about what went wrong or where the money had gone. He said he had wanted to speak in January when he expected to have a fuller grasp of the situation. "I never intended to break any rules or any laws," he said. "I am absolutely hopeful that a full understanding of what happened in the last few days will reveal where those monies are," he said.

Corzine was repeatedly questioned as to where the money went and whether clients' money was used to prop up the collapsing firm, to buy European debt or in other ways that would have violated financial rules.

"I don't know whether this is inadvertent; I don't know whether in the flows of transfers someone held on to some funds," said Corzine. "I don't have the ability other than to speculate where they would be."

Committee member David Scott said it was the "height of disbelief" that an experienced leader such as Corzine could lose over $1bn in client money: "You know nothing about it … We have got to get better answers from you," he said.

His comments will have been closely watched by US authorities and lawyers preparing possible legal actions against him and his firm.

White-collar crime expert Stuart Slotnick said he was surprised Corzine had not opted for the fifth amendment – the constitutional right not to speak in order to avoid self-incrimination. "He may have put his reputation ahead of the advice of his lawyers," said Slotnick. He said any comments Corzine made would now be scrutinised by federal and civil lawyers looking at legal action against the former senator and MF Global.

"There is such tremendous anti-Wall Street mood in the country today that I would have expected him to be more careful. He is probably thinking of his legacy," he said.

In a prepared testimony, Corzine said: "Recognising the enormous impact on many people's lives resulting from the events surrounding the MF Global bankruptcy, I appear at today's hearing with great sadness. My sadness, of course, pales in comparison to the losses and hardships that customers, employees and investors have suffered as a result of MF Global's bankruptcy. Their plight weighs on my mind every day, every hour. And, as the chief executive officer of MF Global at the time of its bankruptcy, I apologise to all those affected."

Corzine said he had "limited access to many relevant documents" since departing from the firm in November: "I simply do not know where the money is."

Corzine quit MF Global days before it was forced to declare bankruptcy, driven to collapse after investors lost confidence following its series of bets on European debt. All 1,066 employees have been fired and a court-appointed trustee is searching for missing funds.

The firm has been accused of being "over leveraged" – relying too heavily on borrowed money, but Corzine said he had reduced the debt at MF Global from $37.3 for every $1 the firm held to "around $30". He admitted the ratio was still far higher than he would have wanted.

He will also defend the firm's trading strategy, which he "strongly advocated."

"I believed that [MF Global's] investments in short-term European debt securities were prudent," Corzine said. "There were discussions at board meetings, at which the transactions were described, analysed and debated."

Before his appearance, the committee had harsh criticism for Gary Gensler, chairman of MF Global regulator CFTC and Corzine's former Goldman Sachs colleague. Gensler was absent from the hearing and has recused himself from the case. "I find that entirely unacceptable," said senator Tim Johnson.

The US justice department and financial regulators are also conducting a search for the missing money. Regulators believe it went missing shortly before the broker's collapse. The CFTC has formally opened an investigation into the broker and issued subpoenas to MF Global and its auditors, PricewaterhouseCoopers.