Police and forensic experts dug through rubble at the site Friday in search of more human remains after bones were discovered as utility crews excavated a manhole there this week.

Some of the remains are as large as arm or leg bones, the Associated Press reported Friday.

Some victims' relatives called for all work at the site to be halted for a new, systematic search and for Congress and the state to investigate.

"These are the bones that these mothers bore," Rosaleen Tallon, whose brother was among the dead in the terrorist attacks, told reporters Friday at a news conference.

WTC Families for Proper Burial said the families that still have not learned of any recovery of their loved ones feel "outrage at the continued cavalier attitude toward the retrieval of human remains" at the site.

"We can no longer rely on accidental discoveries," the group said in a statement.

The group's website says no remains have been found for 1,151 of the 2,749 people who died when two hijacked commercial passenger jets struck the towers, causing them to collapse.

New York Mayor Michael Bloomberg gathered top city officials on Friday for an emergency meeting at city hall to determine why human bones were still being found at the site five years after the attack and what areas of the site should be searched again.

"We'll go out and look at other manholes and other things," Bloomberg said after the meeting.

No identifications made yet

The excavation of the 110-storey twin towers began the evening of the attacks and lasted for nine months. About 20,000 pieces of human remains were found.

The discovery comes just weeks after the fifth anniversary of the attacks.

Earlier this year, construction workers in New York discovered 74 bone fragments from 9-11 victims. The remains were found mixed in with roofing material at the condemned Deutsche Bank building, which stands next to Ground Zero.

The recent material excavated from the manholes was removed from the site without anyone noticing it contained the remains, said utility company Consolidated Edison.

No official identifications have been made yet, said Patrick Brennan, of the New York City Mayor's Community Assistance Unit.

The Port Authority contractor spotted the remains Thursday, the agency said.

Construction workers have been instructed to look for human remains and notify the medical examiner's office when they find them.

With files from the Associated Press