European activist hedge funds succeeded in putting Dutch bank ABN Amro into play. Now their counterparts on the other side of the pond are targeting U.S. financial services companies.

Last week, it was TD Ameritrade which was suddenly feeling the heat, as two hedge funds, JANA Partners LLC and SAC Capital Advisors, pushed the discount brokerage to pursue a merger with E*Trade Financial or Charles Schwab.

Today, it was the turn of Investment Technology Group, which disclosed that hedge fund D.E. Shaw had taken a stake in it and urged the trading technology firm to consider a sale, sending ITG shares climbing more than 7 percent.

Why the sudden interest from activist investors in a sector which had once been considered largely off limits to them? Partially, it simply reflects the growing firepower of activists and their deepening pocketbooks. And they’re also following a path blazed in April by private equity firms J.C. Flowers and Friedman Fleischer & Lowe, which showed through their agreed takeover of Sallie Mae that lenders were as much fair game for buyouts as any other sector.

Certainly, D.E. Shaw signaled it got the message loud and clear. Just a few days before it was jostling ITG to consider a deal, it did a full-fledged takeover deal of its own. The target, yet another company in the financial sector, in this case insurer James River Group, a specialty and casualty insurer.