WASHINGTON, March 19 (Reuters) - A U.S. Senate panel on

Tuesday said it was stepping up its investigation into billions

of dollars in annual airline fees for baggage, seat selection,

ticket changes and other services, demanding airline responses

by next month.

Democratic Senator Richard Blumenthal, who chairs the

Permanent Subcommittee on Investigations, first wrote CEOs of

American Airlines ( AAL ), United Airlines Delta Air

Lines, Spirit Airlines ( SAVE ) and Frontier Airlines in

November seeking answers.

He said Tuesday in new letters the carriers must provide

outstanding information and records by April 2 or it will

"consider alternate means to obtain information needed to

conduct its inquiry."