Shares in online gaming firms have tumbled again - amid reports the US Department of Justice is stepping up its probe into the industry.

It has demanded that investment banks hand over documents and details of phone calls connected with internet gaming firms, reports say.

Partygaming shares fell 7.5% in early Monday trading while Sportingbet and 888.com lost about 3.5%.

Online gambling was effectively banned in the US last year.

Legislation was introduced that outlawed financial transactions related to gaming.

Falling values

Reports on Sunday said that the Department of Justice had ordered 16 banks, including HSBC, Dresdner Kleinwort and Deutsche Bank, as well as accountants and law firms to pass on information.

In July last year, executives from online gaming firms, including Sportingbet and Betonsports were detained by US authorities.

Last week, two founders of internet payment firm Neteller, which has been used by gaming sites, were charged with handling billions of dollars in illegal gambling proceeds.

Millions were wiped off the value of internet gaming firms last year as they ceased US trading.

Before the US crackdown on internet gaming, Partygaming had generated about 75% of its income from the US market while about half of 888 Holding's business came from the country.