WASHINGTON - Election ballots could be safely distributed electronically to Americans overseas, but getting the votes back securely could present problems, a study released yesterday said.

It is difficult to ensure that an electronic ballot is from a registered voter and has not been changed en route, while still protecting the person's privacy, the National Institute of Standards and Technology said in the study. Most ballots now are sent and returned by postal or military mail, which can take time. A few states distribute blank ballots by fax or e-mail.

The study, requested by the Election Assistance Commission, looked at the possibility of handling overseas voting via telephone, fax, e-mail, and the Internet.

Fax, e-mail and the Internet all could be used to deliver blank ballots, the study concluded, significantly reducing delivery times. Procedures could be instituted to reduce any threats to such programs, it said.

But getting the votes back in a secure fashion would be a larger problem that needs to be studied, it said:

Telephone.

Since someone voting by phone could not sign the ballot, officials would have to find another way to authenticate the vote, such as using a personal identification number, though that could be stolen.

Fax.

Returning a ballot by fax raises questions about maintaining voter privacy and how the ballot could be protected from tampering.

E-mail.

Voting by e-mail requires several different computer systems to transfer the ballot. Since the systems would not be controlled by election officials, they could be attacked to tamper with or block movement of the ballot.

Internet.

Voting on a Web page means "a great deal of trust must be placed in the software on the election server to accurately record votes," the report said. It also would rely on computer systems outside the control of election officials.