Yahoo Inc. shareholders who are suing the Internet company face a tall task proving it deliberately sabotaged a proposed deal with Microsoft Corp.

The complaint the investors filed against Yahoo’s board of directors in Delaware Court of Chancery on Monday indicates that board members authorized Yahoo CEO Jerry Yang to issue a press release rejecting an acquisition from a third party a few months before Redmond, Wash.-based Microsoft made its $31 a share, or $44.6 billion offer, to acquire the company. The lawsuit, filed by pension funds Police and Fire Retirement System of Detroit and the General Retirement System of Detroit, also takes aim at a severance package Yahoo adopted in February in event of a change of control.