LONDON (AP) — As U.S. authorities prepare to investigate Silicon Valley's digital giants, they'll look for inspiration — and warnings — from Europe, where regulators have led global efforts to rein in Big Tech with only mixed results.

The European Union's executive commission has slapped Google with multibillion-dollar fines for repeatedly abusing its market dominance to stifle competition. It has also demanded that online companies explain more clearly to users what happens to their personal data, ordered them to pay back billions in taxes and laid out the case for tougher rules covering the digital economy.

National authorities in Britain, Germany, Ireland and France, meanwhile, have mounted their own probes over privacy issues and proposed stricter rules on dangerous content.

Despite these efforts, the big technology companies' power appears undiminished, underscoring the challenge for U.S regulators.