They acted in the wake of speculation in Chubb shares, which saw the price climb from 330p on Wednesday morning to 420p on Thursday when the Chubb management were asked by the Stock Exchange to clarify the company's position.
"The situation is under investigation," said David Peacock, chief executive of Chubb Security plc. "We will know more in due course."
This will be the second probe into Chubb's share dealings. On Friday, the Stock Exchange launched its own inquiry, which will examine trades in Chubb shares in the 24 hours leading up to the announcement.
The Exchange will be using its latest IMAS computer for the investigation. It is designed to detect unusual patterns of share trading and can identify possible insider dealing situations faster than human checks.
Attention is focused on events leading up to the announcement of the bid. Williams first approached Chubb on New Year's Eve. The companies have been in talks since then.
Last Monday, both companies briefed their advisers. On Tuesday, Williams secured a pounds 1bn line of credit to finance the deal, which was scheduled to be announced early next week.
The Office of Fair Trading confirmed that it would look into the merger, which will give Williams a dominant position in the UK locks market through the combination of Chubb with its existing Yale security business. It may seek undertakings from Williams to dispose of parts of Chubb.