Officers are accused of using city computers to equipment to send the offensive e-mails.
A local civil rights group said the e-mails, which allegedly were exchanged among officers in the traffic division, are offensive and racist. The Grand Prairie NAACP said the e-mails contained racial slurs.
"It's the 'n-word,' said Preston Dixon, a member of the Grand Prairie NAACP. "And they called our president's plane a word that is just unacceptable and it said that black people -- using the 'n-word' -- will not hold a job over four years, anyway, so it doesn't matter what the president is right now, give him four years and that 'n- word' won't be there. And so that is just sad in times like these."
Dixon's wife has legal action on an unrelated matter pending against the department.
Grand Prairie police would not say what the e-mails said, but confirmed that the department is conducting an internal investigation.
"I will confirm that the Grand Prairie Police Department is currently investigating some inappropriate e-mails sent on the city's e-mail system," Chief Glen Hill said in an e-mailed statement. "At the conclusion of our internal investigation, we will issue a press release. We will not be issuing any additional comments prior to that time."
Detective John Brimmer, a department spokesman, said another police officer reported the e-mails.
"Keep in mind we're a very large organization, and this just involves a very small handful of officers who actually are good officers, but just made a bad judgment call -- made a serious mistake -- that will certainly be dealt with in the appropriate manner by this police department," he said.
"If they're allowed to come back to the department, i think they should at least be placed on six-month probation," she said.