A member of the Senate Judiciary Committee is asking the nation’s top four wireless carriers to justify the rising rates they charge users to send and receive text messages.

Consumers are paying more than 20 cents per message, up from 10 cents in 2005. This increase “does not appear to be justified by rising costs in delivering text messages.

The senator is concerned that all four of the companies appear to have adopted identical price increases at nearly the same time.

These rate hikes have occurred during the industry’s recent consolidation, which has reduced the number of national wireless carriers in the U.S. to four from six. The large national wireless carriers buy out smaller, regional competitors — as evidenced most recently by Verizon Wireless’ planned acquisition of Alltel Corp.