By Jon Herskovitz

AUSTIN, Texas (Reuters) - Houston-area prosecutors dropped charges on Tuesday against two anti-abortion activists indicted for using illegal government identifications to secretly film a Texas Planned Parenthood facility, saying they could not adequately investigate the case.

David Daleiden and Sandra Merritt were indicted by a grand jury in January and faced up to 20 years in prison after being charged with tampering with a governmental record.

Harris County District Attorney Devon Anderson said in a statement her office was limited in what it can investigate under Texas law due to procedural matters with the grand jury process.

"In light of this and after careful research and review, this office dismissed the indictments," Anderson said, without offering additional information.

Daleiden leads the California-based Center for Medical Progress, which released secretly filmed videos from a Houston-area Planned Parenthood office that appeared to show the women's health group of selling aborted fetal tissue.

Several other videos released last summer also purported to show Planned Parenthood officials trying to negotiate prices for fetal tissue, charges that Planned Parenthood denied. Under federal law, donated human fetal tissue may be used for research, but profiting from its sale is prohibited.

In response to the videos, the Republican leaders of Texas launched a probe of Planned Parenthood.

The Texas investigation based on the allegations from Daleiden's group ended in a twist as a grand jury in January cleared Planned Parenthood of wrongdoing but instead indicted video makers Daleiden and Merritt.

In June, a Texas judge dropped a misdemeanor charge for the pair regarding the purchase of human tissue, a crime that carries a punishment of up to one year in jail.[VnL1N1961JX]

Planned Parenthood has denied Daleiden's allegations and any wrongdoing in the matter. It sued in federal court, arguing that the people who recorded the videos acted illegally.

Planned Parenthood has said Daleiden and Merritt presented fake IDs in April 2015 and posed as research executives from a fictitious company to secretly film conversations at a health and administrative center in Houston.

Lawyers for the pair admitted the two presented fake drivers licenses, saying this was a tactic used by investigative journalists.

(Reporting by Jon Herskovitz; Editing by Alan Crosby, Bernard Orr)