LITTLE ROCK, Ark. (AP) _ Police testified Wednesday they seized a letter to actress Mercedes McCambridge because it was in a briefcase that they thought might contain material relevant to the slayings of her son and his family.

The testimony came from Little Rock police officers in the trial of a lawsuit by Ms. McCambridge to prevent disclosure of the letter from her son, John Markle, of Little Rock.

Markle, 45, killed himself Nov. 16 after shooting his wife, Christine, 45, and daughters Amy, 13, and Suzanne, 9, police say.

Three days before the killings, Markle had been fired from his vice president post at a Little Rock brokerage firm because officials had discovered an undisclosed account under his control.

Ms. McCambridge's lawsuit contends that disclosure of the letter and other documents would be an invasion of privacy, would deprive her of valuable property without just compensation, and violate other constitutional rights.

Police were notified of a problem at the house by attorney Richard L. Lawrence, who said he received a telephone call from Markle about 4 a.m. A briefcase near the door inside the Markle house contained the 13-page letter.

Ms. McCambridge, who won an Academy Award in 1950 as a supporting actress in the film ''All the King's Men,'' filed a lawsuit, as did the Markle estate, because under the Arkansas Freedom of Information Act, the contents of police files become public records when the investigation is closed. Her lawsuit contends the FOI law is unconstitutional.

Police made a copy of the letter to Ms. McCambridge, as well as two letters to Lawrence and a Markle diary, and made them part of the file.

''It's not only Ms. McCambridge's rights at stake here, but the rights of every citizen,'' said Harvey Bell, attorney for Ms. McCambridge, in his opening statement.

Lawrence was the first witness but he broke down and was excused.

William Owen, attorney for Lawrence and the estate, has invoked an attorney-client privilege over the contents of the briefcase, but whether such a privilege exists is one of the issues to be decided in the case, said Lee Munson, the trial judge.

In other testimony, detective Sgt. Eddie Alexander said it is normal police procedure to seize documents, sort through them at police headquarters, make copies of the most important for the case, and return the originals.