Mayor Muriel E. Bowser addresses the media and public before signing the “Sale of Synthetic Drugs Emergency Amendment Act of 2015” into law at the John A. Wilson Building. (Amanda Voisard/For The Washington Post)

The difficulty in testing synthetic drugs is slowing the prosecution of suspects accused of possessing or selling the chemically engineered substances, even as authorities blame them for a spike in violence and overdoses, according to District officials.

Prosecutors with the U.S. attorney’s office have been unable to charge a number of people recently arrested, and many of them have had to be released while officials await test results, city and federal officials said. Police said they hope to charge them once testing is completed.

“It is a little bit of a delay process,” D.C. Police Chief Cathy L. Lanier said at a news conference Friday after Mayor Muriel E. Bowser (D) signed legislation allowing officials to close and fine stores selling synthetic drugs. Assistant Police Chief Peter Newsham said that suspected synthetic drugs that were seized are being sent to a federal Drug Enforcement Administration lab, although he added that testing “doesn’t happen as quickly as we would like.”

Traditional drugs, such as cocaine and heroin, can quickly be detected, so they do not present such testing problems. Suspects are almost always immediately charged with possession or distribution.

But that is not the case with the synthetic varieties, which because of their complicated compounds must be sent to a lab after a drug arrest.

“The District will not tolerate the sale of these drugs and will punish any business selling them,” Mayor Muriel E. Bowser said Friday as she signed emergency legislation. (Amanda Voisard/For The Washington Post)

The charging delays are yet another example of police struggling to keep pace with designer drugs — with such names as Scooby Snax, Bizzaro and Spice — that officials say are made to bypass drug laws and whose manufacturers change formulas as quickly as new rules banning them are put on the books.

“The formulas of the synthetic drugs are constantly being changed to stay a step ahead of law enforcement,” the U.S. attorney’s office said in a statement. “Because the chemicals used in these drugs are constantly changing, and because there is no reliable means of testing those drugs in the field, we cannot sustain charges in these cases until laboratory testing of the compounds is completed.”

The D.C. government and law enforcement agencies have made fighting the problem of synthetic drugs a priority. Nearly a dozen people overdosed last month on suspected synthetic drugs outside a homeless shelter near Judiciary Square. Lanier said that on a recent tour of the city, she encountered three people overdosing — “completely disoriented, disconnected, sometimes in the middle of the street.”

Police have also said they suspect that synthetic drugs were behind two high-profile criminal cases this month — the July 4 killing of a passenger aboard a Metro train in which the victim was stabbed 30 to 40 times during a robbery and the abandonment of a baby near downtown.

But according to D.C. Superior Court documents, the suspect arrested in the Metro attack tested negative for drugs. He was not, however, tested for K2, one of the more popular synthetic drugs linked to psychotic behavior, because it is not part of standard procedure. One law enforcement official said the suspect will be tested for synthetic drugs.

At Friday’s news conference, Newsham said that information about suspected use of synthetic drugs comes from witnesses describing erratic actions “that would suggest folks are using this substance before they are involved in violent criminal behavior.” He added that “there is a strong indication that synthetic cannabinoids are responsible for a lot of the violence we are encountering.”

The legislation that Bowser signed Friday gives Lanierand other regulatory officials emergency powers to close stores and other businesses for 96 hours if authorities determine that they are selling synthetic drugs. Bowser noted the dangers of some of the synthetic drugs, billed as designer marijuana. Instead of offering users a mellow high, they often act more like PCP: They can lead to pyschotic behavior, uncontrolled violent outbursts, seizures and suicidal acts.

“Many of these overdoses have gone unreported and untreated,” Bowser said. “The District will not tolerate the sale of these drugs and will punish any business selling them.”

A first violation carries a $10,000 fine in addition to the 96-hour business closure. A second violation can lead to a 30-day closure, a $20,000 fine and the possibility of a permanent license revocation.

“Make no mistake about it,” Lanier told reporters after the bill was signed. “This drug is not only dangerous to those who use it, it is dangerous to anyone else around them.”

The police chief said the drug is disproportionately affecting impoverished neighborhoods and also is a problem for cities across the country. Lanier pushed for a national strategy “to stop this madness. We don’t want to go back to the crack-cocaine days.” That drug ravaged Washington in the late 1980s and early 1990s and was blamed for the District’s high number of homicides, which turned the nation’s capital into the murder capital.

The chief recently revamped her drug squads to target synthetic drugs that were largely available over the Internet. But, Lanier said, those drugs are “now becoming a street-level drug.” And recent street-level arrests are getting caught up in the complex testing process.

In a letter to Bowser and other officials, U.S. Attorney Vincent H. Cohen Jr. listed five synthetic-drug cases against store owners or employees that are currently being prosecuted. Those include one charged with selling $20,000 of synthetic drugs from a store in Northeast and another who police say had $37,000 of such drugs at a separate shop in Northeast.

Cohen noted in the letter that the DEA has recently identified several new substances as illegal narcotics, and he said officials “will be working collaboratively with the District to get those substances banned ASAP.” In a separate statement, the prosecutor’s office said that police “very recently made a number of arrests following undercover buys, and we are ordering testing in the hopes that we will be able to bring charges in those cases as well.”

Keith L. Alexander and Aaron C. Davis contributed to this report.