Foster was not permitted to discuss his testimony, but he recently accused District Attorney Craig Watkins of failing to investigate allegations against the two constables.
Under Foster's direction, a county worker visited Precinct 5 Constable Jaime Cortes' office early Tuesday morning and copied data from his computer.
"The investigator believes that there's something in there that needs to be turned over to law enforcement, so that's the reason behind this," Foster said.
Foster and two of the five county commissioners voted in September to hire former FBI agent Danny Defenbaugh for a civil investigation of the constables' employment practices after complaining that Watkins was not taking action.
Cortes and Constable Derick Evans also have been accused of improperly accounting for vehicles towed and stored at a South Dallas County wrecker yard. The constables deny any wrongdoing.
It's not clear what the grand jury is investigating. A spokesperson for Watkins' office said Wednesday that district attorney could not comment on any pending grand jury investigation.
The attorney for one of those constables said Wednesday that the DA has turned the tables on Foster.
"I believe that there are some questions before the grand jury and the district attorney as to whether there has been abuse of power by the county judge and his private investigator," said Domingo Garcia, who represents Cortes.
Garcia said Foster violated the law by tampering with Cortes' computer, and Cortes accused Foster of grandstanding.
"I've tried to sit down and talk to Judge Foster and ask him to list out the problems that he has so we can go forward, but he won't do it," Cortes said.
Foster spoke to reporters before his grand jury testimony.
"We're doing what we believe needs to be done and would at no time do anything that would obstruct justice," he said.
Watkins has feuded with commissioners during the past several months over cuts to his budget. In September, he opposed hiring an outside investigator for the constable matter.