WASHINGTON — A former MF Global executive is refusing to answer lawmakers’ questions about more than $1 billion in customer money that vanished in the days before the firm collapsed, invoking her Fifth Amendment right against self-incrimination.

Edith O’Brien, a former assistant treasurer at MF Global, was subpoenaed to testify before the House Financial Services oversight subcommittee hearing about an email she sent in the firm’s final days.

The email appears to contradict testimony from Jon Corzine, the firm’s then-CEO. It says he ordered the transfer of $200 million from a customer account on Oct. 28 to cover an overdraft in the firm’s bank account in London.

Corzine has testified that he never directed anyone to use customer funds to fix the overdraft.