(CNN) -- Prosecutors are seeking additional charges against the Cleveland man accused of keeping three women captive in his home for a decade, they told a judge.
At a eight-minute hearing Wednesday, Ariel Castro, 52, kept his head down, often closing his eyes and speaking only to answer "yes" or "no" to the judge's questions.
Prosecutor Timothy McGinty told Cuyahoga County Judge Michael Russo there is a possibility of more charges.
"We are presenting additional evidence to the grand jury next week and the week after. We expect we are going to request further indictments," he said.
Castro faces 329 counts, including one count of aggravated murder for allegedly causing the unlawful termination of a pregnancy.
Castro's lawyers have filed a speedy-trial motion, which would mean the case would have to be tried by August 4. However, that deadline could change if Castro's attorneys change course and ask for more time to prepare for the case.
Russo set the next pretrial hearing for June 26 and told the prosecution and defense that any plea deal would need to be in writing. The defense has said in the past that it would be open to a plea deal if the death penalty was taken off the table.
This week, Castro entered a not guilty plea to all charges, including 139 counts of rape and 177 counts of kidnapping Michelle Knight, Amanda Berry and Georgina "Gina" DeJesus. Castro fathered a daughter with Berry, according to authorities.
Police have alleged Knight was impregnated by Castro five times, but he would punch her in the stomach, resulting in miscarriages and termination of all the pregnancies.
Authorities said last week they are conducting DNA tests on evidence to determine if additional victims were inside Castro's Cleveland home, and the state attorney general's office says the crime lab is reviewing additional evidence in the case.
Since the case began, investigators have been looking for evidence of aborted fetuses and evidence of any other girls or women who may have been kidnapped or held captive in the house.
CNN's Eliott C. McLaughlin and Mayra Cuevas contributed to this report.