NEW YORK (CNN) -- Hackers have been ringing up big losses at America's top online brokerage houses this year after infiltrating company systems and illegally trading millions of dollars, according to the Securities and Exchange Commission.

All E-Trade (up $0.05 to $22.83, Charts) and TD Ameritrade (down $0.79 to $15.84, Charts) customers who have experienced losses as a result of hackers have been fully reimbursed, the companies said.

Katrina Becker, a spokeswoman for a top-five broker, Ameritrade, confirmed that hackers were the cause of approximately $4 million in losses in that company's third quarter.

At E-Trade Financial, another large broker, spokeswoman Pam Ericson told CNN that hacking had also contributed to her company's third-quarter losses. She would not say what percentage of the company's $18 million loss was incurred by hackers.

A number of federal agencies have responded to the online crime wave, including the SEC, the Secret Service, and the FBI.

John Stark, Chief of the SEC's Office of Internet Enforcement, told CNN hackers have mainly been using two methods to defraud companies. The first involves profiting off the manipulation of stock prices; the second, more simple, method involves the sale of stock and wiring of profits to offshore accounts.

While Becker confirmed that both schemes have been used to defraud Ameritrade, she stressed that they are not the only ones being used. "Criminals will try to do whatever they can," she said. "Just about any type of thing they have tried, we've experienced."

In an effort to combat this type of crime, many large online brokers have begun providing free anti-hacking software to customers, and educating them on the risks of online fraud.

Earlier in the day, the Washington Post reported that Charles Schwab (Charts) customers who experienced losses from hackers will also be reimbursed.

----From CNN's Zak Sos

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