US investigators are still trying to determine a motive for the killing of a teenager and the wounding of her girlfriend in a shooting at a South Texas park that has caught the attention of gay rights groups.

The shooting at a Portland park on Friday night (Saturday NZT) killed 19-year-old Mollie Olgin and wounded 18-year-old Mary Kristene Chapa. Police in Portland, a town of about 15,000 people, have not identified any suspects or announced any arrests.

Family and friends have told police the women were a couple, but police Chief Randy Wright said in a statement late "there is no current evidence to indicate the attacks were motivated by that relationship."

Police have released the initial call report that shows police arrived at the park along the Gulf Coast five minutes after the call came in.

Visitors to the park saw the women in tall grass below an observation deck. Olgin, of Ingleside, was pronounced dead on the scene and Chapa, of Sinton, was taken to a hospital where she was listed in stable condition.

Vigils for the women were scheduled in San Francisco, Washington and Portland, Texas, among other cities.

Gay and lesbian advocacy groups the Human Rights Campaign and Equality Texas condemned the shootings and urged a thorough investigation. Chuck Smith, deputy executive director of Equality Texas, said he was satisfied with the investigation thus far.

"I think on some levels, regardless of the fact that they were gay, it's a horrific crime that touches people just because of the simplest loss of life and that something like that happens in a small town," Smith said.

Jillian Manuel, 20, a friend of Olgin's, echoed that.

"We're trying to figure out why they would hurt Mollie or Kristene," Manuel said.

"Mollie was one of the sweetest girls I ever knew, and from the sounds of it so is Kristene. So we're trying to figure out why someone would just do that because Portland is a safe community. We don't have robberies that go bad. We don't have murder cases like this."

- AP