The family of Natalee Holloway (search) increased the reward Thursday for help finding the Alabama teenager who has been missing for more than six weeks.

There is now a $200,000 reward for her safe return and $100,000 for information that helps authorities solve the mystery surrounding her disappearance, the teen's mother, Beth Holloway Twitty (search), said at a news conference.

Holloway, 18, disappeared May 30, hours before she was to catch a return flight to Mountain Brook, Ala., at the end of high school graduation trip to the Dutch Caribbean island. Extensive searches by Dutch marines, Aruban police, and some 2,000 volunteers have found no trace of her.

"By offering the substantial cash rewards, a plea is made to everyone with any useful information to please call the tip line," Twitty said.

Previously, the family had posted a reward of $175,000 for Holloway's safe return while donors offered more than $50,000 for information on the teen's whereabouts.

The teen's family has also hired a private investigator to help in the search, said Vinda de Sousa, a lawyer for Twitty.

"That does not mean that they have no confidence in the local authorities," de Sousa said.

Holloway was last seen in public leaving a nightclub with Joran van der Sloot, a 17-year-old son of a judge in training on the island, and two Surinamese brothers, Satish and Deepak Kalpoe (search). All three were arrested on June 9 but only van der Sloot remains in custody. No one has been charged.

Authorities took DNA samples from the three on Tuesday, a day after investigators said they would conduct DNA tests on blond hair attached to duct tape that was found along Aruba's northeastern coast. It could take a week or two before the test results are known.