As many as 100 criminals a day are being released back onto the street in the Los Angeles area alone under California’s sanctuary city law, ICE’s top deportation official told Congress on Tuesday as he pleaded for lawmakers to do something.

Timothy S. Robbins, acting associate executive director of detention and removal operations at U.S. Immigration and Customs Enforcement, said his agency used to get 75 to 100 illegal immigrants turned over each day from jails in the Los Angeles region. Now it gets fewer than five a day.

The irony, he said, is that sanctuary communities say they want ICE to focus on deporting criminals — but by refusing to cooperate at their jails they’re actually pushing the agency’s 6,800 deportation officers out into communities, meaning they are more likely to pick up rank-and-file illegal immigrants while arresting fewer criminals.

It also means ICE needs more money and more agents to staff deportation teams that go out into the communities, rather than the one or two officers needed to take custody of people in a jail.

“There’s a true cost,” Mr. Robbins said. “ICE enforcement will no longer be in the jails. It’ll be in the communities — the same communities these sanctuary policies are trying to keep ICE out of. … I will have to send officers out onto the street, which is less safe for officers, the community and the subject at large.”

Mr. Robbins also seemed to stun several Senate Democrats who said they believed sanctuary cities were cooperating when it came to serious criminals, but were refusing to assist ICE only for those with lower-level records.

“Are you saying that local law enforcement, if they knew they had a violent offender in custody, that they would release those persons?” asked Sen. Mazie K. Hirono, Hawaii Democrat.

“Yes, that’s exactly what I’m saying,” Mr. Robbins said. “I have many examples.”

Ms. Hirono was taken aback and repeated her question — twice — in disbelief.

“So your testimony is that the local law enforcement releases violent offenders?”

“That’s what we’re talking about today,” Mr. Robbins told her.

The Washington Times this year has reported on some of those exact types of cases, including an illegal immigrant in Maryland accused of attempted murder who was released in defiance of ICE and who now stands accused of a gang killing, and an MS-13 member in Washington state who notched an increasingly severe criminal record while being shielded from ICE, and who now stands accused of a horrific gang murder.

At one point during Tuesday’s hearing, Sen. Dianne Feinstein, California Democrat, suggested the sanctuary situation in San Francisco that led to the death of Kate Steinle in 2015 — a slaying that helped drive the sanctuary issue to the fore of the debate — was solved.

Mr. Robbins told her that wasn’t so.

“It hasn’t been fixed,” he said.

That was when he revealed the statistics about the Los Angeles area, where he used to work. He said before California’s sanctuary law, SB54, took effect in 2018, ICE was getting 75 to 100 deportable migrants a day from area jails. Today it’s fewer than five.

“All things being equal, there are 70 to 100 criminal aliens hitting the streets in Los Angeles alone. That is one city within the United States. This is a significant problem that has been overlooked for too long,” he said.

The Los Angeles mayor’s office didn’t provide an immediate response to questions.

But Senate Minority Whip Richard J. Durbin, an Illinois Democrat who has become one of his party’s chief voices for immigrant rights, fired back at Mr. Robbins, saying that all lawmakers want to see better cooperation with federal law enforcement, but ICE — and President Trump — have poisoned the well.

Therefore, he said, it comes as no surprise that communities like Chicago refuse to talk to the agency.

“The president scares the hell out of these people that are undocumented,” said Mr. Durbin, using a term activists use to refer to illegal immigrants. “At this point, this president’s declared war on immigrants.”

Mr. Durbin said the solution is legislation to clear the slate and legalize most current illegal immigrants, permanently shielding them from deportation. He pointed to a Chicago case where he said ICE attempted to deport a grandmother, saying that was a major image problem for the agency.

Mr. Robbins countered that grandmothers are not his focus and 70% of his deportation budget is spent on culling criminals out of jails. He said it used to be higher, before the sanctuary city explosion ended cooperation with so many of those jails.

Sanctuary policies come in various forms.

Some local agencies will still notify ICE before release of a target immigrant but won’t hold the person any longer than the local charges call for. That does give ICE a window to come collect the migrant, as long as an officer can get there in time.

Other agencies — such as those in Washington state, in the murder case The Times reported on — refuse even that cooperation.

Mr. Robbins pleaded with Congress to pass legislation making clear that local authorities who hold illegal immigrants for ICE have the blessing of federal law.

Some members of Congress want to pass legislation making cooperation mandatory — with the threat of losing federal money for those agencies that refuse.

Sen. Thom Tillis, the North Carolina Republican who served as chairman for the hearing, said he’s working on another solution. His bill would still allow sanctuary cities to refuse cooperation, as many of them want to do, but would then make them liable to a civil lawsuit from anyone who was victimized by a migrant released under a sanctuary policy.

Sen. Lindsey Graham, South Carolina Republican and chairman of the Judiciary Committee, said the panel will vote on Mr. Tillis’ legislation.

But Tuesday’s hearing was a much about politics as policy — a point underscored by Mr. Graham’s decision to hand over the gavel to Mr. Tillis, who is fighting a primary challenge in North Carolina, and if he survives that contest will face a tricky general election.

His state is ground zero for a major debate over sanctuary cities, with the Democratic governor shooting down efforts to try to force cooperation by communities like Mecklenburg County, home to the city of Charlotte, where the sheriff declines cooperation with ICE’s current detainer system.

R. Andrew Murray, the U.S. attorney in western North Carolina, told senators the sanctuary policies are putting criminals back onto the street.

He pointed to the case of Luis Analberto Pineda-Anchecta, who was arrested in May on assault, kidnapping and strangulation offenses. He was released in defiance of an ICE detainer and then had to be rearrested for the same offenses, the prosecutor said. Along the way, he engaged in an hourslong standoff with a SWAT team.

“They are getting out on the street unnecessarily, and they are reoffending. And they reoffend in the same community that they are comfortable. So if it’s an illegal immigrant, they are going back into an immigrant community and reoffending most of the time,” the prosecutor said.

Mr. Tillis, after the hearing, fired off an email from his campaign demanding to know whether his Republican primary opponent and his likely general election Democratic rival agreed with Mr. Murray.